bug#50390: Improve documentation of 'read-char-choice-use-read-key' and 'y-or-n-p-use-read-key'

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: stefan@marxist.se,  50390@debbugs.gnu.org,  juri@linkov.net
> Date: Sun, 05 Sep 2021 12:13:23 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> I've now added some text to the doc strings of the variables and
> >> functions to try to explain what the difference here is, which isn't
> >> obvious.  Feel free to improve more -- it's kinda hard to succinctly
> >> explain the user interface difference here.
> >
> > What is the non-succinct explanation?
> 
> It's in the patch.  :-)

LGTM.