bug#29400: 26.0; Add Elisp manual index entry for `defvar' to node `Compiler Errors'

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 22 Nov 2017 08:50:33 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> Node `Compiler Errors' seems to be the only place in the Elisp manual
> where we tell users that you can use a vacuous `defvar' (no value) to
> suppress a byte-compiler warning about it not being defined.
> 
> But this node has no index entry - at least none that has the word
> `defvar' in it.  Please add such an entry.

I added some index entries, but I don't understand why you wanted an
index entry with "defvar" in it.  A reader who will look for "defvar"
when they want to find ways of suppressing compiler warnings already
knows that defvar is used for that purpose, so why would they use such
a topic at Info-index's prompt?  That's the opposite of what good
index entries should provide -- they should _lead_ to defvar's
description as the way to suppress warnings when the reader thinks of
"warnings" or some such.

> And please either add the same info (about this use of a vacuous
> `defvar') to node `Defining Variables' or add a cross-reference from
> that node to node `Compiler Errors'.

Done.