Doc: Put \qtvariable in module page back

It was removed by d441d6f39bb846989d95bcf5caf387b42414718d for
some reason.

Change-Id: I333cf71dc5bf69b3a1f5aa04f5089e9fbd617c6b
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>