mpsc docs do not mention the magic 'queue' or 'FIFO' words

[huonw]

The type is a concurrent FIFO queue, but that precise aspect of its behaviour is not made clear in the docs. E.g. the intro to the module could be reworded from

Multi-producer, single-consumer communication primitives threads

to

Multi-producer, single-consumer FIFO communication primitives

and I'm sure there's other places where it is natural to refer to the queue-y-ness of the functionality.