watch_queue: rectify kernel-doc for init_watch()

The command './scripts/kernel-doc -none kernel/watch_queue.c'
reported a mismatch in the kernel-doc of init_watch().

Rectify the kernel-doc, such that no issues remain for watch_queue.c.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>