Define documentation for dropdowns (rules when and how to use it)

[AdrianDampc]

[ePaul]

Now we have both https://fabric-design.github.io/styleguide/#/components/dropdown and https://fabric-design.github.io/styleguide/#/components/03-dropdowns, with overlapping (but not identical) content.

That should be merged, I think – or clearly separated into a designer and developer point of view, with appropriate titles.