Doc: QML_PUPPET_MODE env variable

The variable was added for checking whether a QML plugin
is run in Design mode or by an application.

Task-number: QTCREATORBUG-12439
Change-Id: Iad99b7279c8191cb8b17ca81ff24fb9db731fdc9
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>