Document the existence of addElevatedOperation.

Task-number: QTIFW-407

Change-Id: I885cc8432ceffae48e94f165f415baa0e54155c2
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>