block: Document the bio_vec properties

#include <linux/errno.h>

#include <linux/mm.h>

/*

 * was unsigned short, but we might as well be ready for > 64kB I/O pages

/**

 * struct bio_vec - a contiguous range of physical memory addresses

 * @bv_page:   First page associated with the address range.

 * @bv_len:    Number of bytes in the address range.

 * @bv_offset: Start of the address range relative to the start of @bv_page.

 *

 * The following holds for a bvec if n * PAGE_SIZE < bv_offset + bv_len:

 *

 *   nth_page(@bv_page, n) == @bv_page + n

 *

 * This holds because page_is_mergeable() checks the above property.

 */

struct bio_vec {

	struct page	*bv_page;