[doc] Add doc for public classes and functions of Go packages.

[simonlingoogle]

Besides documentation, there are other changes:

• Some symbols are made private because they are only used in their own packages.
• Some symbols are simply removed because they are useless.
• Some types are moved to more fitting locations.

FYI: Go documenting conventions

Notice this comment is a complete sentence that begins with the name of the element it describes.