Documented dtype=None in tensor.mean()

theano/tensor/basic.py

@@ -2686,8 +2686,10 @@ def mean(input, axis=None, dtype=None, op=False):
     :param dtype: dtype to use for the inner summation. This will not
                   necessarily be the dtype of the output (in particular
                   if it is a discrete (int/uint) dtype, the output will
-                  be in a float type)
-    :type dtype: string
+                  be in a float type).
+                  If None, then we use float64 for a discrete input, and the
+                  same rules as `sum()` for a continuous input.
+    :type dtype: None or string
 
     :note: for gpu, if you specify dtype=float32, everything will be done
            on the gpu.