[PATCH] block: document io scheduler allow_merge_fn hook

				scheduler for example, to reposition the request

				if its sorting order has changed.

elevator_allow_merge_fn		called whenever the block layer determines

				that a bio can be merged into an existing

				request safely. The io scheduler may still

				want to stop a merge at this point if it

				results in some sort of conflict internally,

				this hook allows it to do that.

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