Improve docs for push_data_ready_event and EnsureOmniThread

The push_data_ready_event counter argument is not optional. Also remove mention of the notification daemon since events are handled by ZeroMQ.

Extend description of EnsureOmniThread to include server-side issues.

https://www.tango-controls.org/community/forum/c/development/python/syntax-of-push_data_ready_event/?page=1#post-5136