Add documentation for types and functions

It would be beneficial to add documentation for the interfaces to enable easier usage of the library. The eventual goal would be to provide proper generated documentation via DocC.

Begin by inventorying the library's public interfaces and checking the repository for existing documentation conventions. Add documentation for the relevant types and functions with wording suitable for eventual DocC generation. Done means the public interfaces are documented and the documentation builds if a DocC setup is present.

Written by the indexing model from the issue text.