Minor doc fixes, mostly for timestamp issues

doc/lispref/buffers.texi

@@ -647,7 +647,7 @@ visiting a file or if the time has been explicitly cleared by
 too.  For instance, in a Dired buffer listing a directory, it returns
 the last modification time of that directory, as recorded by Dired.
 
-If the buffer is not visiting a file, this function returns -1.
+If the buffer is not visiting a file, this function returns @minus{}1.
 @end defun
 
 @defun set-visited-file-modtime &optional time
@@ -656,7 +656,8 @@ of the visited file, to the value specified by @var{time} if @var{time}
 is not @code{nil}, and otherwise to the last modification time of the
 visited file.
 
-If @var{time} is neither @code{nil} nor zero, it should have the form
+If @var{time} is neither @code{nil} nor an integer flag returned
+by @code{visited-file-modtime}, it should have the form
 @code{(@var{high} @var{low} @var{microsec} @var{picosec})},
 the format used by @code{current-time} (@pxref{Time of Day}).
 

doc/lispref/files.texi

@@ -1894,8 +1894,7 @@ all.
 This function sets the access and modification times of @var{filename}
 to @var{time}.  The return value is @code{t} if the times are successfully
 set, otherwise it is @code{nil}.  @var{time} defaults to the current
-time and must be in the format returned by @code{current-time}
-(@pxref{Time of Day}).
+time and must be a time value (@pxref{Time of Day}).
 @end defun
 
 @defun set-file-extended-attributes filename attribute-alist

doc/lispref/frames.texi

@@ -2277,7 +2277,7 @@ specifies the color mode to use on a text terminal.  The value can be
 either a symbol or a number.  A number specifies the number of colors
 to use (and, indirectly, what commands to issue to produce each
 color).  For example, @code{(tty-color-mode . 8)} specifies use of the
-ANSI escape sequences for 8 standard text colors.  A value of -1 turns
+ANSI escape sequences for 8 standard text colors.  A value of @minus{}1 turns
 off color support.
 
 If the parameter's value is a symbol, it specifies a number through

doc/lispref/functions.texi

@@ -1629,7 +1629,7 @@ anonymous function.
 This specifies how to order the advice, should several pieces of
 advice be present.  By default, the depth is 0.  A depth of 100
 indicates that this piece of advice should be kept as deep as
-possible, whereas a depth of -100 indicates that it should stay as the
+possible, whereas a depth of @minus{}100 indicates that it should stay as the
 outermost piece.  When two pieces of advice specify the same depth,
 the most recently added one will be outermost.
 

doc/lispref/os.texi

@@ -1457,7 +1457,8 @@ on others, years as early as 1901 do work.
 @cindex formatting time values
 
   These functions convert time values to text in a string, and vice versa.
-Time values are lists of two to four integers (@pxref{Time of Day}).
+Time values include @code{nil}, numbers, and lists of two to four
+integers (@pxref{Time of Day}).
 
 @defun date-to-time string
 This function parses the time-string @var{string} and returns the

doc/lispref/positions.texi

@@ -602,7 +602,7 @@ line @var{count} screen lines from the top of the window; zero means
 the topmost line.  If @var{count} is negative, that specifies a
 position @w{@minus{}@var{count}} lines from the bottom (or the last
 line of the buffer, if the buffer ends above the specified screen
-position); thus, @var{count} of -1 specifies the last fully visible
+position); thus, @var{count} of @minus{}1 specifies the last fully visible
 screen line of the window.
 
 If @var{count} is @code{nil}, then point moves to the beginning of the

doc/lispref/strings.texi

@@ -920,7 +920,7 @@ floating-point number.
 @item %g
 Replace the specification with notation for a floating-point number,
 using either exponential notation or decimal-point notation.  The
-exponential notation is used if the exponent would be less than -4 or
+exponential notation is used if the exponent would be less than @minus{}4 or
 greater than or equal to the precision (default: 6).  By default,
 trailing zeros are removed from the fractional portion of the result
 and a decimal-point character appears only if it is followed by a