Fixes the following warning when running 'make htmldocs':

  include/linux/blk-mq.h:395: warning: Function parameter or member
  'set_rq_budget_token' not described in 'blk_mq_ops'
  include/linux/blk-mq.h:395: warning: Function parameter or member
  'get_rq_budget_token' not described in 'blk_mq_ops'

[mkp: added warning messages]

Link: https://lore.kernel.org/r/20210421154526.1954174-1-ming.lei@redhat.com
Fixes: d022d18c ("scsi: blk-mq: Add callbacks for storing & retrieving budget token")
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>