SUNRPC: Fix comments for transport class registration

The preceding block comment before svc_register_xprt_class() is
not related to that function.

While we're here, add proper documenting comments for these two
publicly-visible functions.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>