bug#41706: 26.1; sort-subr predicate cannot be set successfully

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: Marvin Gülker <post+ebugs@guelker.eu>,
>   41706@debbugs.gnu.org
> Date: Fri, 05 Jun 2020 11:43:19 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > It is hard to be specific about the arguments to PREDICATE, because
> > they depend on what the other arguments to sort-subr return.  I
> > augmented the manual to at least have the same information as the doc
> > string; more detail is only possible if we discuss specific values of
> > STARTKEYFUN and ENDKEYFUN.
> 
> But this sentence is not correct:
> 
> "It [i.e. STARTKEYFUN, added by me] may return either a non-nil value to
> be used as the key, or else the key is the substring between the values
> of point after STARTKEYFUN and ENDKEYFUN are called."

I didn't change that part.

> This is the sentence that misled the OP I think.  Could we be more
> precise and tell that the key is actually the cons cell (BEG . END)
> denoting these strings?

Patches welcome.