Add nested outlet property documentation

[swh-tropic]

Closes #672 by adding documentation on how to properly access nested controller outlets

[raysuelzer]

This seems incorrect, or at least it lacks the html that should go with it. What would I use for this controller

query-builder--query_builder (main controller)

outlet is for controller: query-builder--rule-group

What you have here does not work. The docs say explicitly to OMIT the namespace, but the example ADDs the namespace.