Update ReverbAddEpisodeObserver
Update ReverbAddEpisodeObserver

- Update inaccurate documentation on `reset`. Closing the writer does not write the cached data into the buffer
- Reset the overflow_episode when resetting the observer. Not sure why this wasn't done in the first place.

PiperOrigin-RevId: 327269758
Change-Id: Ic4000f0c4e5e88d471a6afe29b15a753aa96bac0

tensorflow/agents,tensorflow/agents