Fix minor Doxygen issues

Summary:
Fix some linking and "undocumented" issues.

http://ebn.kde.org/apidocs/apidox-extragear/office-kbibtex.html

Test Plan: Create doxygen PDF, new links work as expected

Reviewers: KBibTeX, thomasfischer

Reviewed By: KBibTeX, thomasfischer

Differential Revision: https://phabricator.kde.org/D26335