docs(checkbox): Improve checkbox ngdocs

I've replaced the mentions of the accordion module with the appropriate directive names,
and have also replaced 'Callbacks' with 'Events', and emphasize that these are AngularJS
expressions evaluated with defined locals.

I've also linked to several relevant areas where necessary.
docs(all): Fix documentation and ngdocs build
Merge pull request #8 from caitp/issue-8

Fix the ngdoc comments in checkbox