bug#50364: 27.2; EDT mode Xmodmap related documentation needs updating

[Hin-Tak Leung]

Hi,

Been fighting with EDT mode configuration for a few weeks. 

The fundamental problem is that all the Xmodmap related instruction in
    https://git.savannah.gnu.org/cgit/emacs.git/tree/doc/misc/edt.texi#n418
and the entire section from about
    https://git.savannah.gnu.org/cgit/emacs.git/tree/doc/misc/edt.texi#n325
to
    https://git.savannah.gnu.org/cgit/emacs.git/tree/doc/misc/edt.texi#n530
is significantly outdated, by about a whole decade.

Modern Linux - and also Sun Workstations which have Gnome as desktop - supports
plug-and-play keyboards and mouses with X Keyboard Extension, and would 
periodically
reset any manually applied xmodmap settings; and also since about Gnome 3.6
(around 2010), ignores .Xmodmap .

The proper way of remapping the Num_Lock key to EDT's GOLD key usage
is via a custom keyboard layout with X Keyboard Extension. Failing that,
the Apple family of xkb keyboard models, "Apple laptop", and "Apple Aluminium 
(*)", 
apple_laptop/applealu_ansi/applealu_iso/applealu_jis , have a definition
for the Clear key at that key location.