Mark mandatory elevator functions in the biodoc.txt

biodoc.txt mentions that elevator functions marked with * are mandatory, but
no function is marked with *. Mark the 3 functions which should be
implemented by any io scheduler.
Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

Documentation/block/biodoc.txt

@@ -954,14 +954,14 @@ elevator_allow_merge_fn		called whenever the block layer determines
 				results in some sort of conflict internally,
 				this hook allows it to do that.
 
-elevator_dispatch_fn		fills the dispatch queue with ready requests.
+elevator_dispatch_fn*		fills the dispatch queue with ready requests.
 				I/O schedulers are free to postpone requests by
 				not filling the dispatch queue unless @force
 				is non-zero.  Once dispatched, I/O schedulers
 				are not allowed to manipulate the requests -
 				they belong to generic dispatch queue.
 
-elevator_add_req_fn		called to add a new request into the scheduler
+elevator_add_req_fn*		called to add a new request into the scheduler
 
 elevator_queue_empty_fn		returns true if the merge queue is empty.
 				Drivers shouldn't use this, but rather check
@@ -991,7 +991,7 @@ elevator_activate_req_fn	Called when device driver first sees a request.
 elevator_deactivate_req_fn	Called when device driver decides to delay
 				a request by requeueing it.
 
-elevator_init_fn
+elevator_init_fn*
 elevator_exit_fn		Allocate and free any elevator specific storage
 				for a queue.