Guidebook update

Guidebook update

number_pad got removed from Guidebook.tex when mouse_support was added
by cc0e7a8750fd4e07d3a44592c38f5912d4e603de; put it back.  Evidently
a cut-and-paste mishap when copy-and-paste was intended, probably
provoked by '\numbox{}'.

Remove \numbox{} from mouse_support; it isn't needed there since the
choices aren't trying to line up with '-1'.

Both Guidebook.mn and Guidebook.tex:

Remove number_pad's "for backward compatibility" from mouse_support
since the latter is brand new.

New petattr list of uppercase letters seemed intrusive when rendered
with a bold font, so switch to lowercase.  (Option processing already
supports either case.)

Mention the '=' prefix for hilite_status 'absolute' threshold.

[As usual, the Guidebook.tex changes haven't been tested.]