RDMA/core: Update query_gid documentation for HCA drivers

query_gid() should return right GID value for iWarp and IB link layers.
It is a no-op for RoCE link layer.  Update the documentation to reflect
this.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>