docs: rebasing-and-merging: Drop wrong statement about git

The "^0" syntax is no longer needed to fast-forward to a mainline commit;
take that out and add --ff-only to force an error if fast-forward is not
possible.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
[jc: rewrote changelog]
Link: https://lore.kernel.org/r/20230228134657.1797871-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: Add a document on repository management

Every merge window seems to involve at least one episode where subsystem
maintainers don't manage their trees as Linus would like. Document the
expectations so that at least he has something to point people to.

Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>