bug#36989: 26.2; Doc string of `y-or-n-p': say it returns nil if answer is not `y' or `DEL'

[Drew Adams <drew.adams@oracle.com>]

> > Subject line says it all.  The doc says only that it returns `t' if
> > the answer is `y'.  It should also say what it returns if the answer
> > is no (not `y' or `DEL').
> 
> Did you miss where it says:
>     Return t if answer is "y" and nil if it is "n".
>                                   ^^^^^^^^^^^^^^^^

No, I didn't miss it, because it's not in the Emacs release
I reported on (the latest release, AFAIK): Emacs 26.2.

This is the full doc string (`C-h f'):

y-or-n-p is a compiled Lisp function in 'subr.el'.

(y-or-n-p PROMPT)

Ask user a "y or n" question.  Return t if answer is "y".
Takes one argument, which is the string to display to ask the question.
It should end in a space; 'y-or-n-p' adds '(y or n) ' to it.
No confirmation of answer is requested; a single character is enough.
Also accepts SPC to mean yes, or DEL to mean no.