We want to catch undocumented API before we merge it. The introspection
scanner can fill in the defaults, and this might not cause warnings; but
the documentation is another matter entirely.

Right now, we check only for missing documentation. In the future, we
should add more checks, like:

 - property and signal annotations
 - invalid links
 - legacy gtk-doc content
   - `|[ ... ]|` code blocks
   - `[][]` internal links
   - `%` and `#` sigils

Fixes: #51