Document queues better

[hjoliver]

I've had a few users confused about overlapping queue assignments. The current documentation is a bit too minimal.

The salient points:

Cylc can in principle support different types of queue, but at the moment we only have simple non-overlapping FIFO queues:

• The "default" queue applies to all tasks that have not been assigned to other queues.
• Assigning by family name is just shorthand for assigning the member tasks.
• A given task can only belong to a single queue, so if you assign one to multiple queues only the latest assignment sticks.
• Tasks only get queued when they are ready to run
• Queued tasks get released (on a FIFO basis) when the number of active (submitted or running) members drops under the queue limit

[oliver-sanders]

Suggest documenting this in the queue implementation docstring and autoducmenting it from cylc-doc.

We could also document the base class in the "plugins" section of the docs to assist with queue plugin development. Note we don't have an entry point for queues as yet.