bug#24518: 25.2.50; dired-mark-extension with prefix arg fails

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: tino.calancha@gmail.com,  24518@debbugs.gnu.org
> Date: Sat, 24 Sep 2016 18:48:35 +0200
> 
> > Like I said: it looks a kludge, but it definitely isn't an accident.
> 
> What makes you that confident?  It look certainly like a
> misunderstanding.
> 
> > I suggested to replace 'P' with 'c' in the interactive spec, to make
> > it less kludgey.
> 
> "c" would make it non-optional, definitely not intented.
> 
> >> That doesn't look like this was an intented feature (being
> >> undocumented).
> >
> > It's documented, see the doc string.
> 
> It isn't.  There is no occurence of "prefix" in the doc string.

I believe my recent commit fixed all of the above.