docs: note that extensions.compatobjectformat is incomplete

docs: note that extensions.compatobjectformat is incomplete

The compatibility object format is only implemented for loose objects,
not packed objects, so anyone attempting to push or fetch data into a
repository with this option will likely not see it work as expected.  In
addition, the underlying storage of loose object mapping is likely to
change because the current format is inefficient and does not handle
important mapping information such as that of submodules.

It would have been preferable to initially document that this was not
yet ready for prime time, but we did not do so.  We hinted at the fact
that this functionality is incomplete in the description, but did not
say so explicitly.  Let's do so now: indicate that this feature is
incomplete and subject to change and that the option is not designed to
be used by end users.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>