Add godocs to to the types lib

[fearful-symmetry]

This adds basic godocs to the types lib, so it at least runs linter-clean. I didn't want to do the entire codebase, as touching that much stuff in one PR is a tad annoying, IMO. I wasn't sure about about the verbiage I use for the interfaces, since the way this library uses its interface types is a little odd, as the user is expected to actively cast between interfaces.

[jsoriano]

In general it LGTM, but there is a line that seems incorrect, and I wonder if we should rephrase docs of interfaces a little bit.

[jsoriano]

There is one small typo, once fixed this looks good to go. Thanks!