bug#10854: ^G should be documented as same as n for disabled commands

[jidanni]

Disabled functions now ask
Type y, n, ! or SPC (the space bar):
I think ^G should be documented as a synonym for n.
Therefore say
Type y, n or ^G, ! or SPC (the space bar):

And the *Disable Command* buffer should say
Do you want to use this command anyway?

You can now type
y   to try it and enable it (no questions if you use it again).
n or ^G  to cancel--don't try the command, and it remains disabled.
SPC to try the command just this once, but leave it disabled.
!   to try it, and enable all disabled commands for this session only.