added docstring to NullType

theano/gof/null_type.py

@@ -2,6 +2,15 @@ from type import Type
 from graph import Variable
 
 class NullType(Type):
+    """
+
+    A type that allows no values. Used to represent expressions
+    that are undefined, either because they do not exist mathematically
+    or because the code to generate the expression has not been
+    implemented yet.
+
+    """
+
     def __init__(self, why_null = '(no explanation given)'):
         """
             why_null: A string explaining why this variable