Link from user_type doc to client-sending-data doc

because
http://stackoverflow.com/questions/27060014/how-can-i-pass-a-user-type-from-an-eliom-client-to-a-coservice/27060015
more positive rewording of 344f09a
Merge pull request #155 from unhammer/doc_parameter.ocaml

 Link from user_type doc to client-sending-data doc