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>