res_counter: update documentation

2. Basic accounting routines

 a. void res_counter_init(struct res_counter *rc)

 a. void res_counter_init(struct res_counter *rc,

				struct res_counter *rc_parent)

 	Initializes the resource counter. As usual, should be the first

	routine called for a new counter.

 b. int res_counter_charge[_locked]

			(struct res_counter *rc, unsigned long val)

	The struct res_counter *parent can be used to define a hierarchical

	child -> parent relationship directly in the res_counter structure,

	NULL can be used to define no relationship.

 c. int res_counter_charge(struct res_counter *rc, unsigned long val,

				struct res_counter **limit_fail_at)

	When a resource is about to be allocated it has to be accounted

	with the appropriate resource counter (controller should determine

	  * if the charging is performed first, then it should be uncharged

	    on error path (if the one is called).

 c. void res_counter_uncharge[_locked]

	If the charging fails and a hierarchical dependency exists, the

	limit_fail_at parameter is set to the particular res_counter element

	where the charging failed.

 d. int res_counter_charge_locked

			(struct res_counter *rc, unsigned long val)

	The same as res_counter_charge(), but it must not acquire/release the

	res_counter->lock internally (it must be called with res_counter->lock

	held).

 e. void res_counter_uncharge[_locked]

			(struct res_counter *rc, unsigned long val)

	When a resource is released (freed) it should be de-accounted

	The _locked routines imply that the res_counter->lock is taken.

 2.1 Other accounting routines

    There are more routines that may help you with common needs, like