git-rebase.txt: correct out-of-date and misleading text about renames

Commit 58634db ("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
579c9bb ("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.  While tweaking this description, add a quick
comment about when --merge became the default.

Signed-off-by: Elijah Newren <[email protected]>

Author: newren

Documentation/git-rebase.txt

@@ -340,9 +340,7 @@ See also INCOMPATIBLE OPTIONS below.
 
 -m::
 --merge::
-	Use merging strategies to rebase.  When the recursive (default) merge
-	strategy is used, this allows rebase to be aware of renames on the
-	upstream side.  This is the default.
+	Using merging strategies to rebase (default).
 +
 Note that a rebase merge works by replaying each commit from the working
 branch on top of the <upstream> branch.  Because of this, when a merge