RDMA/ib_srp: Add more documentation

Make it more clear what the different ib_srp data structures represent.

Link: https://lore.kernel.org/r/20220215210511.28303-2-bvanassche@acm.org


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>