Selective documentation generation

[greenh]

Sometimes one wants to generate documentation covering everything---internal/private code as well as public API---and sometimes for APIs only. It'd be interesting to be able to specify that all or parts of specific artifacts were of a particular documentation-class, and then be able to extract docs for only subsets of the universe of documentation classes.

[greenh]

As an addendum, also would like selectivity as to whether or not to include artifacts not containing documentation comments.