Clarify that proto-plus messages are not pickleable, describe the
alternative, and give a multiprocessing example.

Note: in the future, we _may_ enable pickling proto-plus messages
through custom extensions as described in
https://docs.python.org/3/library/pickle.html#pickling-class-instances

This is _not_ guaranteed. For the foreseeable future, the preferred
method of serializing and deserializing proto-plus messages is
Message.serialize and Message.deserialize