message: Fix incorrect documentation for `tf.reduce_all`
diff: @@ -1617,7 +1617,7 @@ def reduce_all(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:
  