message: Update docs for tf.reduce_prod and tf.reduce_mean
diff: @@ -1397,7 +1397,7 @@ def reduce_mean(input_tensor,
  entry in `axis`. If `keepdims` is true, the reduced dimensions
  are retained with length 1.
  - If `axis` has no entries, all dimensions are reduced, and a
  + If `axis` is None, all dimensions are reduced, and a
  tensor with a single element is returned.
  For example:
  @@ -1469,7 +1469,7 @@ def reduce_prod(input_tensor,
  entry in `axis`. If `keepdims` is true, the reduced dimensions
  are retained with length 1.
  - If `axis` has no entries, all dimensions are reduced, and a
  + If `axis` is None, all dimensions are reduced, and a
  tensor with a single element is returned.
  Args:
  