For async send_udp(), return a length and clarify the expiration

For async send_udp(), return a length and clarify the expiration
is meaningless.

For async receive_udp(), document that the return type is always a
3-tuple, as opposed to being either a pair or a 3-tuple as in the
sync version.  (I chose to document rather than change the behavior
as changing it broke dnspython tests and might possibly break other
code.)  [#848]