Doc: several snippet issues in multiple.qdoc

- added path to qdocinc file in qtactiveqt.qdocconf
- removed unnecessary snippet label from multiple-demo.qdocinc
- modified path to qdocinc file
- removed reference to directory name,
  input statement now only specifying file name multiple-demo.qdocinc

Task-number: QTBUG-29101

Change-Id: Icc957907cfcbab97a01ffec07819f667961644b8
Reviewed-by: Martin Smith <martin.smith@digia.com>