bug#43407: 26.3; Please index Escape key (<ESC>) in manuals

[Drew Adams <drew.adams@oracle.com>]

Emacs manual index entries that include "escape" all have to do with
escape sequences or escape chars.  A user looking for info about the
"Escape" keyboard key won't find anything using `i' in the Emacs manual.

Please consider adding an index entry for "Escape key" or some such,
which could take you to node `User Input', for example.

Similarly, for the Elisp manual.  Consider adding index entries that
take you to nodes `Functions for Key Lookup' and `Prefix Keys', the
former, in particular.

Here's a user question that's relevant, together with his comment "I
used to think all along that the escape key was meta key because when I
did Escape - x, spacemacs would show M - x. Why is that the case if
Escape isn't the meta key?"

https://emacs.stackexchange.com/q/60645/105

It should be made easy to find info about the relation between the Meta
modifier key and the Escape prefix key.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''