Fix errors. Clarify ambiguities. Improve presentation. Add cautionary note.

• Removed incorrect note in the task_done docs. That method is not called by shutdown.
• Separate the discussions of the normal case (the default) and the immediate case.
• For the normal case, note that blocked callers of put will be unblocked but will also immediately raise 'ShutDown' in the formerly blocked thread.
• For the normal case, note that get and task_done continue to operate normally except that a get on an empty queue will now raise ShutDown instead of Empty.
• For the immediate case, present the actions in the order that they actually occur (drain the queue, reduce the unfinished tasks by the number drained, notify Queue.join waiters, notify get and put waiters.
• For the immediate case, note that unfinished tasks is updated without calling task_done.
• For the immediate case, note the Queue.join is unblocked even if the unfinished tasks are more than zero.
• Add cautionary note about using Queue.join with immediate=True.

📚 Documentation preview 📚: https://cpython-previews--137028.org.readthedocs.build/