res_counter: update documentation

After the introduction of resource counters hierarchies
(28dbc4b6a01fb579a9441c7b81e3d3413dc452df) the prototypes of
res_counter_init() and res_counter_charge() have been changed.

Keep the documentation consistent with the actual function prototypes.

Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
Cc: Paul Menage <menage@google.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>