Doc: qtgui configuration file with comments

Task-number: QTBUG-31801

Change-Id: I225edf67f586cc5822269c643020d5666465712d
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Tidy up QJsonObject documentation.

Change-Id: I445e0573c3c4fdb86ef535299a4eb299e225c15d
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>