bug#15797: 24.3.50; Info: Mention cache-long-scans

[Eli Zaretskii <eliz@gnu.org>]

> From: Nathan Trapuzzano <nbtrap@nbtrap.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  Michael Heerdegen <michael_heerdegen@web.de>,  15797@debbugs.gnu.org,  kjambunathan@gmail.com
> Date: Fri, 08 Nov 2013 14:07:44 -0500
> 
> Sorry I'm late to the party, but this broke linum and nlinum for me,
> using the defaults.

Any breakage that this does means bugs somewhere in Emacs, because the
same function that invalidates the scan cache when a buffer is
modified also adjusts overlays and markers.  So turning this option on
does us a favor by exposing hidden bugs that are otherwise extremely
hard to reproduce.

> Any time I insert a new line, the line numbers get
> totally messed up--most of them don't even display any more.  And then
> using motion commands sometimes results in really bizarre behavior
> (apparently only with linum/nlinum so far), such as when I do
> forwad-sexp and point goes to the end of some sexp other than the one at
> point.

I cannot reproduce this, and neither can Stefan, so it seems.  Please
provide a recipe for reproducing these problems.

Thanks.