The answer to your question is going to depend a lot on what database platform you are using to contain the synonym; in your question, you referenced documentation from at least two (SQL Server and Informix). I don't know much about Informix, but I'm going to assume that it's security model is different than SQL Server.
For SQL Server, the remote server must be set up as a linked server first (assuming that you are using a remote object). See http://technet.microsoft.com/en-us/library/ms188279.aspx for details on how to do that.