Item documentation: visibleChildren is a list of items, not a real

Followup to I5ec541226fabd72c05ce8ccb8bb7e56f6ec7717a

Task-number: QTBUG-22724
Change-Id: I3f74fda29faba4054c5bb97aad4735f7c5f0f278
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>