update API doc strings in framework for public-facing APIs

[gregcaporaso]

Many of these either don't have doc strings, or have some formatting in the doc strings that doesn't align with either the Numpy or Google conventions. As part of this effort, we should choose whether we prefer the Numpy or Google convention and use that throughout.

This work will primarily happen in the QIIME 2 framework codebase, though there should be content updates in DWQ2 as new API documentation is produced.