rebase: restore help for rebase w/o args (issue5059)

Restoring documentation accidentally removed in a9a047878e14.

hgext/rebase.py

@@ -121,7 +121,7 b' def rebase(ui, repo, **opts):'
     destination changeset is not modified by rebasing, but new
     changesets are added as its descendants.)
 
-    There are three ways to select changesets::
+    Here are the ways to select changesets::
 
       1. Explicitly select them using ``--rev``.
 
@@ -131,6 +131,9 b' def rebase(ui, repo, **opts):'
       3. Use ``--base`` to select a changeset; rebase will find ancestors
       and their descendants which are not also ancestors of the destination.
 
+      4. If you do not specify any of ``--rev``, ``source``, or ``--base``,
+      rebase will use ``--base .`` as above.
+
     Rebase will destroy original changesets unless you use ``--keep``.
     It will also move your bookmarks (even if you do).