Improve indexing of modifier keys

* doc/emacs/commands.texi (User Input): Add index entry for
the Alt key serving as Meta.
* doc/emacs/custom.texi (Modifier Keys): Add index entries for
Alt, Super, and Hyper modifier keys.  (Bug#38315)

doc/emacs/commands.texi

@@ -42,6 +42,7 @@ are certain characters found on non-English keyboards
 @cindex C-
 @cindex META
 @cindex M-
+@cindex Alt key, serving as Meta
   Emacs also recognizes control characters that are entered using
 @dfn{modifier keys}.  Two commonly-used modifier keys are
 @key{Control} (usually labeled @key{Ctrl}), and @key{Meta} (usually
@@ -64,6 +65,8 @@ next character; instead, press @key{ESC} and release it, then enter
 the next character.  This feature is useful on certain text terminals
 where the @key{Meta} key does not function reliably.
 
+  Emacs supports 3 additional modifier keys, see @ref{Modifier Keys}.
+
 @cindex keys stolen by window manager
 @cindex window manager, keys stolen by
   On graphical displays, the window manager might block some keyboard

doc/emacs/custom.texi

@@ -1919,6 +1919,12 @@ characters case-sensitive (even on non-graphical frames) when you
 customize Emacs.  For instance, you could make @kbd{M-a} and @kbd{M-A}
 run different commands.
 
+@cindex Alt, modifier key
+@cindex Super, modifier key
+@cindex Hyper, modifier key
+@cindex s-
+@cindex H-
+@cindex A-
   Although only the @key{Control} and @key{Meta} modifier keys are
 commonly used, Emacs supports three other modifier keys.  These are
 called @key{Super}, @key{Hyper}, and @key{Alt}.  Few terminals provide