Improve documentation of Diff mode

* doc/emacs/files.texi (Diff Mode): Document the effect of
prefix argument on the Diff mode's commands.  Document
'diff-jump-to-old-file'.

doc/emacs/files.texi

@@ -1427,23 +1427,30 @@ manually, type @kbd{M-x diff-mode}.
 @cindex hunk, diff
   The changes specified in a patch are grouped into @dfn{hunks}, which
 are contiguous chunks of text that contain one or more changed lines.
-Hunks can also include unchanged lines to provide context for the
+Hunks usually also include unchanged lines to provide context for the
 changes.  Each hunk is preceded by a @dfn{hunk header}, which
-specifies the old and new line numbers at which the hunk occurs.  Diff
+specifies the old and new line numbers where the hunk's changes occur.
-mode highlights each hunk header, to distinguish it from the actual
+Diff mode highlights each hunk header, to distinguish it from the
-contents of the hunk.
+actual contents of the hunk.
+
+  The first hunk in a patch is preceded by a file header, which shows
+the names of the new and the old versions of the file, and their time
+stamps.  If a patch shows changes for more than one file, each file
+has such a header before the first hunk of that file's changes.
 
 @vindex diff-update-on-the-fly
   You can edit a Diff mode buffer like any other buffer.  (If it is
-read-only, you need to make it writable first.  @xref{Misc Buffer}.)
+read-only, you need to make it writable first; see @ref{Misc Buffer}.)
-Whenever you change a hunk, Diff mode attempts to automatically
+Whenever you edit a hunk, Diff mode attempts to automatically correct
-correct the line numbers in the hunk headers, to ensure that the patch
+the line numbers in the hunk headers, to ensure that the patch remains
-remains correct.  To disable automatic line number correction,
+correct, and could still be applied by @command{patch}.  To disable
-change the variable @code{diff-update-on-the-fly} to @code{nil}.
+automatic line number correction, change the variable
+@code{diff-update-on-the-fly} to @code{nil}.
 
-  Diff mode treats each hunk as an error message, similar to
+  Diff mode arranges for hunks to be treated as compiler error
-Compilation mode.  Thus, you can use commands such as @kbd{C-x `} to
+messages by @kbd{C-x `} and other commands that handle error messages
-visit the corresponding source locations.  @xref{Compilation Mode}.
+(@pxref{Compilation Mode}).  Thus, you can use the compilation-mode
+commands to visit the corresponding source locations.
 
   In addition, Diff mode provides the following commands to navigate,
 manipulate and apply parts of patches:
@@ -1451,7 +1458,8 @@ manipulate and apply parts of patches:
 @table @kbd
 @item M-n
 @findex diff-hunk-next
-Move to the next hunk-start (@code{diff-hunk-next}).
+Move to the next hunk-start (@code{diff-hunk-next}).  With prefix
+argument @var{n}, move forward to the @var{n}th next hunk.
 
 @findex diff-auto-refine-mode
 @cindex mode, Diff Auto-Refine
@@ -1469,19 +1477,22 @@ default, add this to your init file (@pxref{Hooks}):
 
 @item M-p
 @findex diff-hunk-prev
-Move to the previous hunk-start (@code{diff-hunk-prev}).  Like
+Move to the previous hunk-start (@code{diff-hunk-prev}).  With prefix
+argument @var{n}, move back to the @var{n}th previous hunk.  Like
 @kbd{M-n}, this has the side-effect of refining the hunk you move to,
 unless you disable Diff Auto-Refine mode.
 
 @item M-@}
 @findex diff-file-next
 Move to the next file-start, in a multi-file patch
-(@code{diff-file-next}).
+(@code{diff-file-next}).  With prefix argument @var{n}, move forward
+to the start of the @var{n}th next file.
 
 @item M-@{
 @findex diff-file-prev
 Move to the previous file-start, in a multi-file patch
-(@code{diff-file-prev}).
+(@code{diff-file-prev}).  With prefix argument @var{n}, move back to
+the start of the @var{n}th previous file.
 
 @item M-k
 @findex diff-hunk-kill
@@ -1496,7 +1507,10 @@ In a multi-file patch, kill the current file part.
 @findex diff-apply-hunk
 @cindex patches, applying
 Apply this hunk to its target file (@code{diff-apply-hunk}).  With a
-prefix argument of @kbd{C-u}, revert this hunk.
+prefix argument of @kbd{C-u}, revert this hunk, i.e.@: apply the
+reverse of the hunk, which changes the ``new'' version into the ``old''
+version.  If @code{diff-jump-to-old-file} is non-@code{nil}, apply the
+hunk to the ``old'' version of the file instead.
 
 @item C-c C-b
 @findex diff-refine-hunk
@@ -1506,8 +1520,16 @@ of each changed line were actually changed.
 
 @item C-c C-c
 @findex diff-goto-source
+@vindex diff-jump-to-old-file
 Go to the source file and line corresponding to this hunk
-(@code{diff-goto-source}).
+(@code{diff-goto-source}).  By default, this jumps to the ``new''
+version of the file, the one shown first on the file header.
+With a prefix argument, jump to the ``old'' version instead.  If
+@code{diff-jump-to-old-file} is non-@code{nil}, this command by
+default jumps to the ``old'' file, and the meaning of the prefix
+argument is reversed.  If the prefix argument is a number greater than
+8 (e.g., if you type @kbd{C-u C-u C-c C-c}), then this command also
+sets @code{diff-jump-to-old-file} for the next invocation.
 
 @item C-c C-e
 @findex diff-ediff-patch
@@ -1517,41 +1539,47 @@ Start an Ediff session with the patch (@code{diff-ediff-patch}).
 @item C-c C-n
 @findex diff-restrict-view
 Restrict the view to the current hunk (@code{diff-restrict-view}).
-@xref{Narrowing}.  With a prefix argument of @kbd{C-u}, restrict the
+@xref{Narrowing}.  With a prefix argument, restrict the
 view to the current file of a multiple-file patch.  To widen again,
 use @kbd{C-x n w} (@code{widen}).
 
 @item C-c C-r
 @findex diff-reverse-direction
 Reverse the direction of comparison for the entire buffer
-(@code{diff-reverse-direction}).
+(@code{diff-reverse-direction}).  With a prefix argument, reverse the
+direction only inside the current region (@pxref{Mark}).  Reversing
+the direction means changing the hunks and the file-start headers to
+produce a patch that would change the ``new'' version into the ``old''
+one.
 
 @item C-c C-s
 @findex diff-split-hunk
-Split the hunk at point (@code{diff-split-hunk}).  This is for
+Split the hunk at point (@code{diff-split-hunk}) into two separate
-manually editing patches, and only works with the @dfn{unified diff
+hunks.  This inserts a hunk header and modifies the header of the
-format} produced by the @option{-u} or @option{--unified} options to
+current hunk.  This command is useful for manually editing patches,
-the @command{diff} program.  If you need to split a hunk in the
+and only works with the @dfn{unified diff format} produced by the
-@dfn{context diff format} produced by the @option{-c} or
+@option{-u} or @option{--unified} options to the @command{diff}
-@option{--context} options to @command{diff}, first convert the buffer
+program.  If you need to split a hunk in the @dfn{context diff format}
-to the unified diff format with @kbd{C-c C-u}.
+produced by the @option{-c} or @option{--context} options to
+@command{diff}, first convert the buffer to the unified diff format
+with @kbd{C-c C-u}.
 
 @item C-c C-d
 @findex diff-unified->context
 Convert the entire buffer to the @dfn{context diff format}
 (@code{diff-unified->context}).  With a prefix argument, convert only
-the text within the region.
+the hunks within the region.
 
 @item C-c C-u
 @findex diff-context->unified
 Convert the entire buffer to unified diff format
 (@code{diff-context->unified}).  With a prefix argument, convert
 unified format to context format.  When the mark is active, convert
-only the text within the region.
+only the hunks within the region.
 
 @item C-c C-w
 @findex diff-ignore-whitespace-hunk
-Re-diff the current hunk, disregarding changes in whitespace
+Re-generate the current hunk, disregarding changes in whitespace
 (@code{diff-ignore-whitespace-hunk}).
 
 @item C-x 4 A
@@ -1582,7 +1610,8 @@ that whitespace in both the patch and the patched source file(s).
 This command does not save the modifications that it makes, so you can
 decide whether to save the changes (the list of modified files is
 displayed in the echo area).  With a prefix argument, it tries to
-modify the original source files rather than the patched source files.
+modify the original (``old'') source files rather than the patched
+(``new'') source files.
 
 @node Copying and Naming
 @section Copying, Naming and Renaming Files