Add warning about interpolating values from groups

Add a warning explaining that options in groups cannot be used with
the interpolation syntax.

Reformat note to use directive so it is highlighted.

Add "docs" section to tox.ini to make testing doc build easier.

Change-Id: If5106aa44d4c87abe84a7fb7a250541bba9f5068
Merge "Add warning about interpolating values from groups"