Update iv_wait.3 man page for recent API additions.
Update iv_wait.3 man page for recent API additions.

Adds documentation for iv_wait_interest_register_spawn() and
iv_wait_interest_kill().

Signed-off-by: Lennert Buytenhek <8858498612e3a522354216649776e3032dcb7e5b@wantstofly.org>

buytenh/ivykis,buytenh/ivykis