bug#21746: 24.5; purpose of dired-keep-marker-copy?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 24 Oct 2015 23:01:12 -0500
> From: "Roland Winkler" <winkler@gnu.org>
> Cc: Andreas Schwab <schwab@linux-m68k.org>, 21746@debbugs.gnu.org
> 
> > How else could Dired distinguish the copied files?
> 
> It shouldn't.  Marks are for new actions in the future, not for old
> ones from the past.
> 
> If you nonetheless like inconsistent behavior, don't make it the
> default.

We are arguing here about the default value of a defcustom.  Moreover,
this value was introduced 23 years ago, never changed since then, and
I don't remember any complaints about that default (but if you find
any, please point to them).

So it doesn't sound wise to me to change that default based on the
fact that you, Roland, dislike it.  You can always customize this
variable (and other similar dired-keep-marker-* options) to you
heart's content, right?  OTOH, changing this long-standing default
would be an incompatible change, something that we only do when the
community as a whole requests.  Which is not the case here, I think.

Thus, I think we should document this subtlety, both in the doc
strings of the corresponding dired-do-* commands and in the user
manual, and otherwise leave the existing default alone.

Patches for the documentation are welcome.