Document the "format": "reference" parameters type

[fdrozdowski]

It seems they were implemented as part of this effort: 

https://jira.delphix.com/browse/DLPX-40412

https://docs.google.com/presentation/d/10unw_UA5-XgQF_YGcWPUtiXpRIU4MMgtydzXUYunWhE/edit#slide=id.g6772f927a_0_0 

I verified the parameter type works with python toolkits and return the reference strings for the environment and the user which are selected in the parameters.

┆Issue is synchronized with this Jira Bug by Unito