block: fix parameter not described warning

The build warning:
block/blk-core.c:968: warning: Function parameter or member 'iob'
not described in 'bio_poll'.

Fixes: 5a72e899 ("block: add a struct io_comp_batch argument to fops->iopoll()")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -1017,6 +1017,7 @@ EXPORT_SYMBOL(submit_bio);
 /**
  * bio_poll - poll for BIO completions
  * @bio: bio to poll for
+ * @iob: batches of IO
  * @flags: BLK_POLL_* flags that control the behavior
  *
  * Poll for completions on queue associated with the bio. Returns number of