Missing documentation that QLayout constructor sets parent layout

The constructor of QLayout sets the parent widget's layout to 'this' (see here). The bug is that this behavior is not documented. For example here:

http://doc.qt.io/qt-5/qlayout.html#QLayout

The "Basic Layouts Example" page does not mention why it creates Layouts without parent widgets, either:

http://doc.qt.io/qt-5/qtwidgets-layouts-basiclayouts-example.html

 This can lead to a difficult to debug warning:

QWidget::setLayout: Attempting to set QLayout "" on Widget "", which already has a layout

 when the following happens:

• Creating a layout not intended for widget A with it as parent (to avoid a potential memory leak):

QVBoxLayout *mainLayout = new QVBoxLayout(A);

• And giving that widget another layout later:

A->setLayout(anotherLayout);

For an example of this affecting users in the wild, see for example:

https://stackoverflow.com/questions/10519006/qwidgetsetlayout-attempting-to-set-qlayout-on-widget-which-already-has

https://gitlab.orfeo-toolbox.org/orfeotoolbox/otb/merge_requests/187