bio_add_page() and __bio_add_page() are capable of adding pages into
bio, and now we have at least two such usages alreay:

	- __bio_iov_bvec_add_pages()
	- nvmet_bdev_execute_rw().

So update comments on these two helpers.

The thing is a bit special for __bio_try_merge_page(), given the caller
needs to know if the new added page is same with the last added page,
then it isn't safe to pass multi-page in case that 'same_page' is true,
so adds warning on potential misuse, and updates comment on
__bio_try_merge_page().

Cc: linux-xfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>