missing documentation of `command_list_ok_begin` & `command_list_end`

[jankaifer]

These two commands are essential when issuing multiple commands (like adding 2000 sogs to a playlist). But I was not able to find any mention of them in the documentation of python-mpd2.

[Mic92]

It's implemented https://github.com/Mic92/python-mpd2/blob/master/mpd/base.py#L700 Feel free to add it to the documentation. In the past we used to generate the documentation using the upstream protocol spec.