Documentation: Add nested versions of mutex locks to docs

Signed-off-by: Robert P. J. Day <[e-mail]>
Signed-off-by: Adrian Bunk <[e-mail]>

Documentation/mutex-design.txt

···
  int  mutex_trylock(struct mutex *lock);
  void mutex_unlock(struct mutex *lock);
  int  mutex_is_locked(struct mutex *lock);
-
+ void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
+ int  mutex_lock_interruptible_nested(struct mutex *lock,
+                                      unsigned int subclass);