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

[Lars Ingebrigtsen <larsi@gnus.org>]

Juri Linkov <juri@linkov.net> writes:

>> When I look into the documentation of those variables, I do not get
>> any explanation of why I would want to use them.  I also can't find
>> any explanation in the manual.
>>
>> Even just one or two sentences that indicates its typical use would be
>> very helpful here.
>
> The problem is that read-char-choice still has no docstring.
> After adding it, the docstring of read-char-choice should
> refer to read-char-choice-use-read-key and explain it.
>
> y-or-n-p already has docstring and should mention y-or-n-p-use-read-key.

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.

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