block: Update blk_update_request() documentation

Although the original intent was to use blk_update_request() in stacking
block drivers only, it is used much more widely today. Reflect this in the
documentation block above this function. See also:
* commit 32fab448e5 ("block: add request update interface").
* commit 2e60e02297 ("block: clean up request completion API").
* commit ed6565e734 ("block: handle partial completions for special
  payload requests").

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20210519175226.8853-1-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1384,26 +1384,22 @@ void blk_steal_bios(struct bio_list *list, struct request *rq)
 EXPORT_SYMBOL_GPL(blk_steal_bios);
 
 /**
- * blk_update_request - Special helper function for request stacking drivers
+ * blk_update_request - Complete multiple bytes without completing the request
  * @req:      the request being processed
  * @error:    block status code
- * @nr_bytes: number of bytes to complete @req
+ * @nr_bytes: number of bytes to complete for @req
  *
  * Description:
  *     Ends I/O on a number of bytes attached to @req, but doesn't complete
  *     the request structure even if @req doesn't have leftover.
  *     If @req has leftover, sets it up for the next range of segments.
  *
- *     This special helper function is only for request stacking drivers
- *     (e.g. request-based dm) so that they can handle partial completion.
- *     Actual device drivers should use blk_mq_end_request instead.
- *
  *     Passing the result of blk_rq_bytes() as @nr_bytes guarantees
  *     %false return from this function.
  *
  * Note:
- *	The RQF_SPECIAL_PAYLOAD flag is ignored on purpose in both
- *	blk_rq_bytes() and in blk_update_request().
+ *	The RQF_SPECIAL_PAYLOAD flag is ignored on purpose in this function
+ *      except in the consistency check at the end of this function.
  *
  * Return:
  *     %false - this request doesn't have any more data