[PATCH] lockdep: add more rwsem.h documentation

Add more documentation to rwsem.h.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>