Document the semantics of receiveWait and similar

Currently the documentation reads:

-- | Test the matches in order against each message in the queue
receiveWait :: [Match b] -> Process b Source

This is not sufficient, I think, to understand how receiveWait works in the presence of multiple messages in the mailbox and multiple match clauses. The order in which matches are done should be stated less succinctly.

I suppose we should also say that this function:

• blocks when no message matches
• it is interruptible when no message matches (can it be interrupted even if some message matches?).

Start at the receiveWait API documentation and inspect the surrounding documentation for similar receive functions. Confirm the mailbox and match-clause ordering, blocking when no message matches, and interruptibility, including the case where some messages match. Done means these semantics are stated clearly in the documentation.