push doc: document the DWYM behavior pushing to unqualified <dst>

push doc: document the DWYM behavior pushing to unqualified <dst>

Document the DWYM behavior that kicks in when pushing to an
unqualified <dst> reference.

This behavior was added in f88395ac23 ("Renaming push.", 2005-08-03)
and f8aae12034 ("push: allow unqualified dest refspecs to DWIM",
2008-04-23), and somewhat documented in bb9fca80ce ("git-push: Update
description of refspecs and add examples", 2007-06-09), but has never
been fully documented.

The closest we got to having documented it was the description in the
commit message for f8aae12034, which I've borrowed from in writing
this documentation.

Let's also refer to this new documentation from the existing
documentation we had (added in bb9fca80ce).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>