[PATCH] Doc/lockdep-design: explain display of {state-bits}

- 'ever used'                                       [ == !unused        ]

When locking rules are violated, these 4 state bits are presented in the

locking error messages, inside curlies.  A contrived example:

   modprobe/2287 is trying to acquire lock:

    (&sio_locks[i].lock){--..}, at: [<c02867fd>] mutex_lock+0x21/0x24

   but task is already holding lock:

    (&sio_locks[i].lock){--..}, at: [<c02867fd>] mutex_lock+0x21/0x24

The bit position indicates hardirq, softirq, hardirq-read,

softirq-read respectively, and the character displayed in each

indicates:

   '.'	 acquired while irqs enabled

   '+'  acquired in irq context

   '-'  acquired in process context with irqs disabled

   '?'  read-acquired both with irqs enabled and in irq context

Unused mutexes cannot be part of the cause of an error.

Single-lock state rules:

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