Add comment following code review.

theano/tensor/opt_uncanonicalize.py

@@ -44,6 +44,8 @@ def local_max_and_argmax(node):
     """
     if node.op == T._max_and_argmax:
         if len(node.outputs[1].clients) == 0:
+            #MaxAndArgmax support variable axis,
+            #but CAReduce support only constant axis.
             try:
                 axis = get_scalar_constant_value(node.inputs[1])
             except NotScalarConstantError: