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.deSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/maintainer/rebasing-and-merging.rst

@@ -213,11 +213,7 @@ point rather than some random spot.  If your upstream-bound branch has
 emptied entirely into the mainline during the merge window, you can pull it
 forward with a command like::
 
-  git merge v5.2-rc1^0
-
-The "^0" will cause Git to do a fast-forward merge (which should be
-possible in this situation), thus avoiding the addition of a spurious merge
-commit.
+  git merge --ff-only v5.2-rc1
 
 The guidelines laid out above are just that: guidelines.  There will always
 be situations that call out for a different solution, and these guidelines