locking/ww_mutex/Documentation: Update the design document

  normal mutex locks, which are far more common. As such there is only a small

  increase in code size if wait/wound mutexes are not used.

  We maintain the following invariants for the wait list:

  (1) Waiters with an acquire context are sorted by stamp order; waiters

      without an acquire context are interspersed in FIFO order.

  (2) Among waiters with contexts, only the first one can have other locks

      acquired already (ctx->acquired > 0). Note that this waiter may come

      after other waiters without contexts in the list.

  In general, not much contention is expected. The locks are typically used to

  serialize access to resources for devices. The only way to make wakeups

  smarter would be at the cost of adding a field to struct mutex_waiter. This

  would add overhead to all cases where normal mutexes are used, and

  ww_mutexes are generally less performance sensitive.

  serialize access to resources for devices.

Lockdep:

  Special care has been taken to warn for as many cases of api abuse