bug#63557: 26.3; Doc string of `find-file-run-dired'

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

> > Instead of just saying that non-nil allows `find-file' to visit
> > directories, say also (as does (emacs) `Visiting') that otherwise (i.e.,
> > nil) raises an error when the arg is a directory name.
>
> How would you interpret "allows" in any other sense?  What could "does
> not allow" mean when the value is nil?
> IOW, I don't see why would we need to spoon-feed the user in this
> case.  The meaning of nil is crystal clear to me.

That you can't visit a directory is one thing.
That trying to do that raises an error is a
different thing.  Another possible behavior
that doesn't visit a directory is to do nothing.
A third is to show a message, without raising
an error.

Not being able to do something is not the same
as having an error raised if you try to do it.