[KAddressbook] Documentation improves

Summary: Manual revision

Reviewers: KDE PIM, vkrause, yurchor, mlaurent, ognarb

Reviewed By: KDE PIM, yurchor, mlaurent, ognarb

Subscribers: ognarb, davidbryant, kde-pim, yurchor, kde-doc-english, vkrause

Tags: KDE PIM, Documentation

Differential Revision: https://phabricator.kde.org/D25253