Doc changes for cache-long-scans

Ref: http://debbugs.gnu.org/15797#65

* doc/lispref/display.texi (Truncation):
* doc/lispref/positions.texi (Screen Lines): Do not mention cache-long-scans.

* src/buffer.c (cache-long-scans): Doc fix.

* etc/NEWS: Related edit.

doc/lispref/ChangeLog

@@ -1,3 +1,8 @@
+2014-02-06  Glenn Morris  <rgm@gnu.org>
+
+	* display.texi (Truncation):
+	* positions.texi (Screen Lines): Do not mention cache-long-scans.
+
 2014-01-31  Juri Linkov  <juri@jurta.org>
 
 	* searching.texi (String Search): Incremental word search fixes.

doc/lispref/display.texi

@@ -214,6 +214,7 @@ A line prefix may also be specified for regions of text using the
 over the @code{line-prefix} variable.  @xref{Special Properties}.
 @end defvar
 
+@ignore
   If your buffer contains only very short lines, you might find it
 advisable to set @code{cache-long-scans} to @code{nil}.
 
@@ -227,6 +228,7 @@ Turning off the cache speeds up processing of short lines somewhat.
 
 This variable is automatically buffer-local in every buffer.
 @end defvar
+@end ignore
 
 @node The Echo Area
 @section The Echo Area

doc/lispref/positions.texi

@@ -486,9 +486,11 @@ flag, and display table may vary between windows).  @xref{Usual
 Display}.
 
   These functions scan text to determine where screen lines break, and
-thus take time proportional to the distance scanned.  If you intend to
-use them heavily, Emacs provides caches which may improve the
-performance of your code.  @xref{Truncation, cache-long-scans}.
+thus take time proportional to the distance scanned.
+@ignore
+If you intend to use them heavily, Emacs provides caches which may
+improve the performance of your code.  @xref{Truncation, cache-long-scans}.
+@end ignore
 
 @defun vertical-motion count &optional window
 This function moves point to the start of the screen line @var{count}

etc/NEWS

@@ -240,9 +240,9 @@ to be printed in other formats (octal, hexadecimal, and character).
 ** `write-region-inhibit-fsync' now defaults to t in batch mode.
 
 +++
-** `cache-long-line-scans' has been renamed to `cache-long-scans'
+** `cache-long-line-scans' is now non-nil, and renamed to `cache-long-scans',
 because it affects caching of paragraph scanning results as well.
-Also, it is now enabled by default.
+There is no reason to set this to nil except for debugging purposes.
 
 ---
 ** The option `set-mark-default-inactive' has been deleted.

src/ChangeLog

@@ -1,3 +1,7 @@
+2014-02-06  Glenn Morris  <rgm@gnu.org>
+
+	* buffer.c (cache-long-scans): Doc fix.
+
 2014-02-05  Eli Zaretskii  <eliz@gnu.org>
 
 	* w32fns.c (Fw32_shell_execute): Doc fix.

src/buffer.c

@@ -6130,6 +6130,8 @@ If the value of the variable is t, undo information is not recorded.  */);
   DEFVAR_PER_BUFFER ("cache-long-scans", &BVAR (current_buffer, cache_long_scans), Qnil,
 		     doc: /* Non-nil means that Emacs should use caches in attempt to speedup buffer scans.
 
+There is no reason to set this to nil except for debugging purposes.
+
 Normally, the line-motion functions work by scanning the buffer for
 newlines.  Columnar operations (like `move-to-column' and
 `compute-motion') also work by scanning the buffer, summing character