Clarify how to use custom typeref return types for Action methods

[evanw555]

Currently, the documentation mentions that Action methods can return "Custom types", but that's about it. It's not clear that in order to do so, the returnTyperef attribute of the @Action annotation must be set to a valid typeref class.

Location in the documentation: https://linkedin.github.io/rest.li/user_guide/restli_server#action

This will be somewhat clarified by the changes made in #496

e.g.

@Action(name = "doAction", returnTyperef = CustomLongRef.class)
public CustomLong doAction() {
  return new CustomLong(1L);
}