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

[Eli Zaretskii]

> From: Jambunathan K <kjambunathan@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  Michael Heerdegen 
> <michael_heerdegen@web.de>,  15797@debbugs.gnu.org
> Date: Wed, 06 Nov 2013 10:14:37 +0530
> 
> Stefan Monnier <monnier@iro.umontreal.ca> writes:
> 
> > Let's try it.
> 
> Please don't hide the variable in lispref.  

Given that the variable is now on by default, and I see no reason why
anyone would might want to turn it off, except fro debugging, does it
still make sense to have this advertised in the user manual?  If so,
why?