Document desired behavior for fiber::thread_affinity().
Also clarify Preconditions for fiber::priority().
Document new [un]bounded_queue API, plus a bit of guessing.

Change certain [un]bounded_queue method signatures to return queue_op_status.
Their previous signatures didn't provide any way to report a closed queue.
For Note: interruption point, document Throws: fiber_interrupted.
Write a few more words about bounded_queue's high water mark and low water
mark.
Document that bounded_queue constructor can in fact throw.
Describe this_fiber::thread_affinity() in terms of fiber::thread_affinity().
Also mention that fiber::thread_affinity() is off by default.
Fix markup error in fiber::thread_affinity() Note.
Start adding internal cross-references to fiber documentation