dm: document iterate_devices

typedef int (*dm_merge_fn) (struct dm_target *ti, struct bvec_merge_data *bvm,

			    struct bio_vec *biovec, int max_size);

/*

 * These iteration functions are typically used to check (and combine)

 * properties of underlying devices.

 * E.g. Does at least one underlying device support flush?

 *      Does any underlying device not support WRITE_SAME?

 *

 * The callout function is called once for each contiguous section of

 * an underlying device.  State can be maintained in *data.

 * Return non-zero to stop iterating through any further devices.

 */

typedef int (*iterate_devices_callout_fn) (struct dm_target *ti,

					   struct dm_dev *dev,

					   sector_t start, sector_t len,

					   void *data);

/*

 * This function must iterate through each section of device used by the

 * target until it encounters a non-zero return code, which it then returns.

 * Returns zero if no callout returned non-zero.

 */

typedef int (*dm_iterate_devices_fn) (struct dm_target *ti,

				      iterate_devices_callout_fn fn,

				      void *data);