nvme-fabrics: document the poll function argument

This patch avoids that the kernel-doc tool reports a warning when
building with W=1.

Fixes: 26c68227

 ("nvme-fabrics: allow nvmf_connect_io_queue to poll") # v5.0-rc1
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>