xprtrdma: Add documenting comments

Clean up: fill in or update documenting comments for transport
switch entry points.

For xprt_rdma_allocate:

The first paragraph is no longer true since commit 5a6d1db4
("SUNRPC: Add a transport-specific private field in rpc_rqst").

The second paragraph is no longer true since commit 54cbd6b0
("xprtrdma: Delay DMA mapping Send and Receive buffers").

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>