D26918: Improve KNotification API docs

[Nicolas Fella]

nicolasfella added a reviewer: jucato.

REPOSITORY
  R289 KNotifications

REVISION DETAIL
  https://phabricator.kde.org/D26918

To: nicolasfella, #frameworks, broulik, jucato
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns