RDMA/core/multicast: Provide description for 'ib_init_ah_from_mcmember()'s 'rec' param

Fixes the following W=1 kernel build warning(s):

 drivers/infiniband/core/multicast.c:739: warning: Function parameter or member 'rec' not described in 'ib_init_ah_from_mcmember'

Link: https://lore.kernel.org/r/20210118223929.512175-12-lee.jones@linaro.org
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/multicast.c

@@ -721,6 +721,7 @@ EXPORT_SYMBOL(ib_sa_get_mcmember_rec);
  * member record and gid of the device.
  * @device:	RDMA device
  * @port_num:	Port of the rdma device to consider
+ * @rec:	Multicast member record to use
  * @ndev:	Optional netdevice, applicable only for RoCE
  * @gid_type:	GID type to consider
  * @ah_attr:	AH attribute to fillup on successful completion