bug#73387: 30.0.90; C-x v v in diff-mode doesn't work after C-c C-n

[Eli Zaretskii <eliz@gnu.org>]

> From: Sean Whitton <spwhitton@spwhitton.name>
> Cc: dgutov@yandex.ru,  73387@debbugs.gnu.org,  monnier@iro.umontreal.ca,
>   juri@linkov.net
> Date: Thu, 03 Oct 2024 08:50:37 +0800
> 
> >   @item C-c @key{RET} k
> >   Delete diff hunks other than the current one.  If the region is
> >   active, this command deletes the hunks overlapped by the region;
> >   otherwise it deletes all the hunks other than the current hunk.  Do
> >   @emph{not} invoke this command in a narrowed buffer, as that could
> >   produce unexpected results.
> 
> Okay, I've rewritten based on this, though I haven't included the
> imprecation, because invoking it in a narrowed buffer could never
> produce unexpected results -- the function will always do nothing.

Then why do we signal user-error in that case?  I think this should be
explained in the manual.