[PyQt] Small doc fix

[Elvis Stansvik]

Hi Phil,

A small fix for:

    http://pyqt.sourceforge.net/Docs/PyQt5/qt_properties.html

would be to change:

    notify – the optional unbound notify signal. It is ignored by Python

to

    notify – the optional unbound notify signal. Only needed if you
want to use the property in QML property bindings

since it seems I must use it if I want to use the property in QML
property bindings, e.g:

Window {
    color: machine.initializing ? "red" : "green"
    ...
}

or else I'll get errors like:

    QQmlExpression: Expression
file:///home/estan/Projekt/orexplore/dev/src/orexplore.machine/orexplore/hmi/qml/main.qml:15:12
depends on non-NOTIFYable properties: Machine::initializing

Cheers,
Elvis