bug#37925: 26.3; Elisp manual: add index entry for sets/kinds of variables

[Drew Adams]

These nodes are about particular kinds of variables:

* Buffer-Local Variables
* Completion Variables
* Condition Variables
* Connection Local Variables
* Constant Variables
* Directory Local Variables
* File Local Variables
* Generalized Variables
* Global Variables
* List Variables
* List Variables
* Local Variables
* Mode Line Variables
* Other Font Lock Variables
* Output Variables
* Variables with Restricted Values
* Void Variables
* Warning Variables

Likewise, for options:

* Warning Options
* Choosing Window Options
* Edebug Options
* Network Options

Please consider adding index entries that correspond directly to these
node names.  A user should be able to do, for example (and preferably
with or without the hyphen):

  i mode-line variables

Today that's not possible.

---

[Additional remark:
 The node names are not consistent wrt compound adjectives.
 `Buffer-Local' is the only such that uses a hyphen.  It's better to be
 consistent.  (Even better might be to always use a hyphen.)]


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.17763
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''