Adds kerneldoc for inet_twsk_unhash() & inet_twsk_bind_unhash().

With help from Randy Dunlap.
Suggested-by: NEvgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>