git-rebase.txt: correct out-of-date and misleading text about renames

git-rebase.txt: correct out-of-date and misleading text about renames

Commit 58634dbff8 ("rebase: Allow merge strategies to be used when
rebasing", 2006-06-21) added the --merge option to git-rebase so that
renames could be detected (at least when using the `recursive` merge
backend).  However, git-am -3 gained that same ability in commit
579c9bb198 ("Use merge-recursive in git-am -3.", 2006-12-28).  As such,
the comment about being able to detect renames is not particularly
noteworthy.  Remove it.

Acked-by: Derrick Stolee <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>