message: Update optimizer doc.
diff: @@ -415,8 +415,8 @@ class OptimizerV2(trackable.Trackable):
  passed to the `Optimizer` constructor.
  Returns:
  - An `Operation` that applies the specified gradients. If `global_step`
  - was not None, that operation also increments `global_step`.
  + An `Operation` that applies the specified gradients. The `iterations`
  + will be automatically increased by 1.
  Raises:
  TypeError: If `grads_and_vars` is malformed.
  