Fix minor Doxygen issues

Summary:
Minor fixes (typos, formatting, missing items).

The list of issues is from EBN:

http://ebn.kde.org/apidocs/apidox-kde-4.x/kdeutils-kgpg.html

Test Plan: Run doxywizard. Ensure that there are no warnings except for mismatched class members.

Reviewers: dakon

Reviewed By: dakon

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D17795