btrfs: document more parameters of submit_extent_page

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2746,7 +2746,19 @@ static int __must_check submit_one_bio(struct bio *bio, int mirror_num,
 
 /*
  * @opf:	bio REQ_OP_* and REQ_* flags as one value
+ * @tree:	tree so we can call our merge_bio hook
+ * @wbc:	optional writeback control for io accounting
+ * @page:	page to add to the bio
+ * @pg_offset:	offset of the new bio or to check whether we are adding
+ *              a contiguous page to the previous one
+ * @size:	portion of page that we want to write
+ * @offset:	starting offset in the page
+ * @bdev:	attach newly created bios to this bdev
  * @bio_ret:	must be valid pointer, newly allocated bio will be stored there
+ * @end_io_func:     end_io callback for new bio
+ * @mirror_num:	     desired mirror to read/write
+ * @prev_bio_flags:  flags of previous bio to see if we can merge the current one
+ * @bio_flags:	flags of the current bio to see if we can merge them
  */
 static int submit_extent_page(unsigned int opf, struct extent_io_tree *tree,
 			      struct writeback_control *wbc,