D6789: API dox: add note about moc include need with K_PLUGIN_FACTORY(_WITH_JSON)

[Michael Pyne]

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

REPOSITORY
  R244 KCoreAddons

BRANCH
  addnoteaboutmocincludewithpluginfactory

REVISION DETAIL
  https://phabricator.kde.org/D6789

To: kossebau, #frameworks, mpyne