Document select_loop!'s looping even more explicitly
Document select_loop!'s looping even more explicitly

Attempt to improve `select_loop!`s documentation, making it even more
explicit that the loop will end after the first selection is made.

Hopefully this will prevent the confusion described in #27.

crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,aturon/crossbeam,crossbeam-rs/crossbeam