bug#67620: Better indent-rigidly prompt

[Dan Jacobson <jidanni@jidanni.org>]

What does "Indent region with TAB" mean?
To the layman it means put tabs at the beginning of each line.

But here it means something different:
Indent region with TAB, <left>, <right>, S-<left>, S-<right>

So maybe indent-rigidly should prompt with
Indent region with <left>, <right>, S-<left>, S-<right>, or TAB

In fact that is their order in the docstring!

Else one stares at the prompt for 30 seconds tying to figure out what
it is trying to say.

Or maybe say
Type TAB, <left>, <right>, S-<left>, S-<right>, to indent region
or
Type <left>, <right>, S-<left>, S-<right>, or TAB to indent region

Also it seems C-x C-i is easier to type than C-x TAB. So maybe mention
that one can type either. (Yes, same bytes sent, at least on a VT100.)