Documentation for custom types

* create a new type
- walk through all options for creating a specific type
* scenario:
- create a new type and a new module with a parameter of that type
- show how completions and validation work
* @jeskew please fill in any gaps you see here

Start from the issue's custom-type requirements and identify the documentation location covering type creation. Document the available options, then add the scenario showing a custom type used as a module parameter, including completions and validation. Done means the requested walkthrough and scenario are complete, with gaps reviewed by @jeskew.

Written by the indexing model from the issue text.