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

[Drew Adams]

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.

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'.

As it is now, it is not so easy to find this info.



In GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32)
 of 2017-10-13
Repository revision: 906224eba147bdfc0514090064e8e8f53160f1d4
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''