dlm: add comments about forced waiters reset

When a lock is waiting for a reply for a remote operation, and recovery
interrupts this "waiters" state, the remote operation is voided by the
recovery, and no reply will be processed.  The lkb waiters state for the
remote operation is forcibly reset/cleared, so that the lock operation
can be restarted after recovery.  Improve the comments describing this.

Signed-off-by: David Teigland <teigland@redhat.com>