Documents and refactors loader.h

This pull request fixes the following things:

  * The non-rendered code block in `docs/overview.md`.
  * Uses javadoc-style autobriefs for documentation. The first sentence of a doc comment is now the brief.
  * Refactors `loader.h` to include more descriptive names and move some structures from `loader.c` over for documentation purposes.
  * Documents all functions provided by `loader.h`.