SUNRPC: Modify synopsis of rpc_client_register()

The rpc_client_register() helper was added in commit e73f4cc0,
"SUNRPC: split client creation routine into setup and registration,"
Mon Jun 24 11:52:52 2013.  In a subsequent patch, I'd like to invoke
rpc_client_register() from a context where a struct rpc_create_args
is not available.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>