You will have to convert your data list to a string representation. You could convert it with a mapping to a JSON or XML string. See MF below for an example, model share didn't work.
Regarding the documentation:
If you are using the REST module than the documentation page is generated automatically (see for example https://deploy.mendix.com/api/)
If you want to add some custom text you can set the attribute Documentation as described here https://github.com/mendix/RestServices#core-configuration
Hope this helps,