revset: added docstring to baseset class

revset: added docstring to baseset class

--- a/mercurial/revset.py	Mon Feb 10 00:54:40 2014 +0100
+++ b/mercurial/revset.py	Thu Feb 06 11:33:36 2014 -0800
@@ -2043,6 +2043,9 @@
         return funcs
 
 class baseset(list):
+    """Basic data structure that represents a revset and contains the basic
+    operation that it should be able to perform.
+    """
     def __init__(self, data):
         super(baseset, self).__init__(data)
         self._set = None