block, bfq: clarify the goal of bfq_split_bfqq()

The exact, general goal of the function bfq_split_bfqq() is not that
apparent. Add a comment to make it clear.
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -5979,6 +5979,8 @@ static void bfq_finish_requeue_request(struct request *rq)
 }
 
 /*
+ * Removes the association between the current task and bfqq, assuming
+ * that bic points to the bfq iocontext of the task.
  * Returns NULL if a new bfqq should be allocated, or the old bfqq if this
  * was the last process referring to that bfqq.
  */