D6773: Add API dox for KDEInstallDirs' KDE_INSTALL_USE_QT_SYS_PATHS

[Aleix Pol Gonzalez]

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

REPOSITORY
  R240 Extra CMake Modules

BRANCH
  adddoxforqtsyspaths

REVISION DETAIL
  https://phabricator.kde.org/D6773

To: kossebau, #frameworks, apol
Cc: #build_system