Add a WARNING for sleep, and expand documentation.
Add a WARNING for sleep, and expand documentation.

sleep doesn't really work on GHC because it is always immediately
interrupted by SIGVTALRM used in the RTS.

I explained the problem in a comment and added a WARNING pragma.

usleep and nanosleep have a similar problem, but, since they have better
precision, they can be restarted, so they are not as unusable as sleep.

jimenezrick/unix,rimmington/unix,jimenezrick/unix,rimmington/unix,thomie/unix,rimmington/unix,rimmington/unix,thomie/unix