[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