Edit Briefcase/Aggregate API page for clarity

briefcase-api.rst

The info here is good, and (seems) complete, but is hard to read and unclear. (More like a brain dump than a narrative.)

Part of #402

Start by reading briefcase-api.rst and identify how its existing information is organized. Reshape the page into a clearer narrative while preserving the complete API information; done means the Briefcase/Aggregate API documentation is easier to read and understand.

Written by the indexing model from the issue text.