fixed some comments in scan

theano/scan.py

@@ -304,6 +304,13 @@ def scan(fn, sequences=[], outputs_info=[], non_sequences=[],
           of -1
         * if you wrap an output in a dictionary but you do not provide any initial state,
           it assumes that you are not using any form of taps
+        * if you provide a ``None`` scan assumes that you will not use any taps for this 
+          output
+
+        If you did not provide any information for your outputs, scan will assume by default
+        that you are not using any taps for any of the input. If you provide information for
+        just a subset of outputs, scan will not know to which outputs these information 
+        corresponds and will raise an error.
 
     :param non_sequences:
         Parameters over which scan should not iterate.  These parameters are