bug#19608: 25.0.50; Elisp manual: add index entry for major modes killing local variables

[Eli Zaretskii]

> Date: Thu, 15 Jan 2015 08:56:01 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> I've seen a few times now the question posed as to why local variables
> that have been set become unset.  Users who asked sometimes looked for
> info in the Elisp manual but didn't find it.
> 
> Please consider adding an index entry that might help users get there
> when looking up local variable information.  Dunno what the best such
> entry/entries might be.  Perhap something like:
> 
> "local variables, killed automatically" or "local variables, killed by
> major modes".  Anyway, you get the idea.

Done.