Merry Christmas all!

Is it just me, or does the Control.Concurrent.MVar documentation seem a bit
misleading?  In particular, we should explicitly note the race conditions
for not just swapMVar but also readMVar, withMVar, modifyMVar_ and modifyMVar,
and clarify that the safety guarantees of the latter three pertain to their
handling of asynchronous exceptions.

It might also be good to tell people that if they need race-free operations
of this style, STM is a good alternative to look at, even if only one variable
is being synchronized over.