undo C-_ and C-/: mention hex codes

[jidanni@jidanni.org]

In the emacs manual at the several places where it talks about the
undo keys C-_ and C-/, please also mention what hex codes you are
talking about (0x1F) etc. Maybe you are just talking about the same
0x1F and 0x1F, or maybe two different codes. The user wants to know.

Or maybe there's a extra layer of abstraction involved. Mention that
too.

Anyways, mentioning it would help the user better understand
paragraphs like:

      The reason the `undo' command has three key bindings, `C-x u', `C-_'
   and `C-/', is that it is worthy of a single-character key, but `C-x u'
   is more straightforward for beginners to remember and type.  Meanwhile,
   `C--' on a text-only terminal is really `C-_', which makes it a natural
   and easily typed binding for undoing.