added func tags

doc/extending/op.txt

@@ -162,8 +162,8 @@ following methods:
   NullType for that input. Likewise, if you have not implemented the
   grad computation for some input, you may return a variable of type
   NullType for that input. theano.gradient contains convenience methods
-  that can construct the variable for you: grad_undefined and
-  grad_not_implemented, respectively.
+  that can construct the variable for you: :func:`grad_undefined` and
+  :func:`grad_not_implemented`, respectively.
 
   If an element of output_gradient is of type theano.gradient.DisconnectedType,
   it means that the cost is not a function of this output. If any of the