bug#38315: 26.3; Emacs manual: please index Alt key for node User Input

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

There are index entries for the Alt key on MS Windows, but there is no
index entry for node `User Input', where it is mentioned in a general
context (not just MS Windows).  Please add an index entry for this.

These are the current index entries containing "alt" that refer to an
Alt key:

<Alt>-<TAB> vs M-<TAB> (MS-Windows)
<AltGr> key (MS-Windows)
Alt key (MS-Windows)
Alt key invokes menu (Windows)
AltGr (MS-Windows)
M-<TAB> vs <Alt>-<TAB> (MS-Windows)

ns-alternate-modifier
ns-right-alternate-modifier
w32-alt-is-meta
w32-pass-alt-to-system
w32-recognize-altgr

(I had to look for "meta" in the index to find the mention of the Alt
key in node `User Input'.)

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''