block: update comment of "struct bio_vec" regarding nth_page()

Ever since commit 858c708d9e ("block: move the bi_size update out of
__bio_try_merge_page"), page_is_mergeable() no longer exists, and the
logic in bvec_try_merge_page() is now a simple page pointer comparison.

Link: https://lkml.kernel.org/r/20250901150359.867252-37-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/bvec.h

@@ -22,11 +22,8 @@ struct page;
  * @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.
+ * All pages within a bio_vec starting from @bv_page are contiguous and
+ * can simply be iterated (see bvec_advance()).
  */
 struct bio_vec {
 	struct page	*bv_page;