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

[Drew Adams]

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.

Here is one such question: http://emacs.stackexchange.com/q/7391/105.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'