Document PYSIDE-315 implications

Document about why dynamically adding signals and slots (on signal connection for instance) will cause issues both in Qt and PySide, and might lead to hard-to-debug issues and crashes.

Main takeaways:
1) Always decorate slots with @QtCore.Slot()
2) Don't dynamically add / remove signals or slots after a connection is made to any signal or slot.
3) Do not create signals in __init__ methods, the mySignal = QtCore.Signal() definitions should be as class members of the custom class.

2023 addition :
4) For QObject's registered with QML, always use @Slot() for invokable methods, the automatic method registration can fail (PYSIDE-2436)

Also "Add a logging category to libpyside" bd6a2f1f880effb95ea3e42f52d06f0730103a03 https://codereview.qt-project.org/c/pyside/pyside-setup/+/426960 adds a logging category to get respective warnings by

export QT_LOGGING_RULES="qt.pyside.libpyside.warning=true".