bug#57529: 28.1; `C-c C-x' prefix key and keymap, `C-c C-x C-h'

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

Dunno, but this seems like a bug (you decide):

https://emacs.stackexchange.com/q/73390/

`C-c' is a prefix key bound to `mode-specific-map', but it seems to be
set up in C source code.  `C-c C-x' is apparently defined (unlike, say,
`C-c C-x 4') as a prefix key, but it's not clear what its keymap is (and
it's apparently empty by default).

Dunno what changes should be made, to make clear to users what the story
is - why, for example, `C-c C-x' just waits for another key, even though
there's no such key defined in the keymap.

Maybe this is a more general "problem", for empty prefix keys?  Or is it
a result of the prefix being defined in C?

In GNU Emacs 28.1 (build 2, x86_64-w64-mingw32)
 of 2022-04-21 built on AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19044
System Description: Microsoft Windows 10 Pro (v10.0.2009.19044.1889)

Configured using:
 'configure --with-modules --without-dbus --with-native-compilation
 --without-compress-install CFLAGS=-O2'

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY W32NOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
XPM ZLIB

(NATIVE_COMP present but libgccjit not available)

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252