Improve documentation for undiscovered QLEService

The class' behavior strongly deponds on its state. This can be
confusing. This patch reduces the behavior uncertainties.

Change-Id: Ib5cd8e821f2bb9aca5b579abf4ae98918f951f24
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>