D28986: API dox: document Q_DECLARE_FLAGS-based flags

[David Faure]

dfaure accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R244 KCoreAddons

BRANCH
  documentFlags

REVISION DETAIL
  https://phabricator.kde.org/D28986

To: kossebau, #frameworks, dfaure
Cc: kde-frameworks-devel, LeGast00n, cblack, michaelh, ngraham, bruns