message: Add performance note for recursive methods.
  Addresses comments from tensorflow/community#56.
diff: @@ -201,6 +201,10 @@ class Module(six.with_metaclass(ModuleMetaclass, tracking.AutoCheckpointable)):
  def variables(self):
  """Collection of variables owned by this module and it's submodules.
  + Note: this method uses reflection to find variables on the current instance
  + and submodules. For performance reasons you may wish to cache the result
  + of calling this method if you don't expect the return value to change.
  +
  Returns:
  A collection of variables for the current module (sorted by attribute
  name) followed by variables from all submodules recursively (depth first).
  @@ -225,6 +229,10 @@ class Module(six.with_metaclass(ModuleMetaclass, tracking.AutoCheckpointable)):
  def trainable_variables(self):
  """Collection of variables owned by this module and it's submodules.
  + Note: this method uses reflection to find variables on the current instance
  + and submodules. For performance reasons you may wish to cache the result
  + of calling this method if you don't expect the return value to change.
  +
  Returns:
  A collection of variables for the current module (sorted by attribute
  name) followed by variables from all submodules recursively (depth first).
  