six locks: Documentation, renaming

 - Expanded and revamped overview documentation in six.h, giving an
   overview of all features
 - docbook-comments for all external interfaces
 - Rename some functions for simplicity, i.e.
   six_lock_ip_type() -> six_lock_ip()
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_locking.h

@@ -205,8 +205,8 @@ static inline int __btree_node_lock_nopath(struct btree_trans *trans,
 	trans->lock_must_abort	= false;
 	trans->locking		= b;
 
-	ret = six_lock_type_ip_waiter(&b->lock, type, &trans->locking_wait,
-				   bch2_six_check_for_deadlock, trans, ip);
+	ret = six_lock_ip_waiter(&b->lock, type, &trans->locking_wait,
+				 bch2_six_check_for_deadlock, trans, ip);
 	WRITE_ONCE(trans->locking, NULL);
 	WRITE_ONCE(trans->locking_wait.start_time, 0);
 	return ret;