rebase: attempt to clarify --base

hgext/rebase.py

@@ -50,10 +50,9 b' def _makeextrafn(copiers):'
 
 @command('rebase',
     [('s', 'source', '',
-     _('rebase from the specified changeset'), _('REV')),
+     _('rebase the specified changeset and descendants'), _('REV')),
     ('b', 'base', '',
-     _('rebase the tree around the specified changeset without '
-       'ancestors of dest'),
+     _('rebase everything from branching point of specified changeset'),
      _('REV')),
     ('r', 'rev', [],
      _('rebase these revisions'),