Update iv_work(3) man page for addition of thread start/stop callbacks.
Update iv_work(3) man page for addition of thread start/stop callbacks.

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

buytenh/ivykis,buytenh/ivykis