rebase --rebase-merges: adjust man page for octopus support

Now that we support octopus merges in the `--rebase-merges` mode,
we should give users who actually read the manuals a chance to know
about this fact.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

Documentation/git-rebase.txt

@@ -879,8 +879,8 @@ rescheduled immediately, with a helpful message how to edit the todo list
 (this typically happens when a `reset` command was inserted into the todo
 list manually and contains a typo).
 
-The `merge` command will merge the specified revision into whatever is
-HEAD at that time. With `-C <original-commit>`, the commit message of
+The `merge` command will merge the specified revision(s) into whatever
+is HEAD at that time. With `-C <original-commit>`, the commit message of
 the specified merge commit will be used. When the `-C` is changed to
 a lower-case `-c`, the message will be opened in an editor after a
 successful merge so that the user can edit the message.
@@ -889,10 +889,12 @@ If a `merge` command fails for any reason other than merge conflicts (i.e.
 when the merge operation did not even start), it is rescheduled immediately.
 
 At this time, the `merge` command will *always* use the `recursive`
-merge strategy, with no way to choose a different one. To work around
-this, an `exec` command can be used to call `git merge` explicitly,
-using the fact that the labels are worktree-local refs (the ref
-`refs/rewritten/onto` would correspond to the label `onto`, for example).
+merge strategy for regular merges, and the `octopus` strategy for
+octopus merges, strategy, with no way to choose a different one. To work
+around this, an `exec` command can be used to call `git merge`
+explicitly, using the fact that the labels are worktree-local refs (the
+ref `refs/rewritten/onto` would correspond to the label `onto`, for
+example).
 
 Note: the first command (`label onto`) labels the revision onto which
 the commits are rebased; The name `onto` is just a convention, as a nod