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.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 33bd80d3f4f..cbf7291e3c8 100644
--- a/src/buffer.c
+++ b/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