rebase: mention --rev in the help

This changeset adds a small mention of it in the help to prevent
confusion. This small addition references online help that is easier to
update and improve at release time.

Following Wagner Bruna's advice, this is added in a plain new paragraph
to not invalidate current translation this close to the release.

hgext/rebase.py

@@ -88,6 +88,11 b' def rebase(ui, repo, **opts):'
     the whole branch. If you specify neither ``-s`` nor ``-b``, rebase
     uses the parent of the working directory as the base.
 
+    For advanced usage, a third way is available through the ``--rev``
+    option. It allows you to specify an arbitrary set of changesets to
+    rebase. Descendants of revs you specify with this option are not
+    automatically included in the rebase.
+
     By default, rebase recreates the changesets in the source branch
     as descendants of dest and then destroys the originals. Use
     ``--keep`` to preserve the original source changesets. Some