message: Add 'existing' to parse's docstring
diff: @@ -521,7 +521,7 @@ class HParams(object):
  del self._hparam_types[name]
  def parse(self, values):
  - """Override hyperparameter values, parsing new values from a string.
  + """Override existing hyperparameter values, parsing new values from a string.
  See parse_values for more detail on the allowed format for values.
  