Clarify how to override automatic plugin generation

https://doc.qt.io/qt-6/qqmlengineextensionplugin.html says that, if we manually subclass QQmlEngineExtensionPlugin, we must pass NO_GENERATE_PLUGIN_SOURCE to the qt_add_qml_module(). However, this alone is insufficient because qt_add_qml_module() separates the backing target and the plugin target by default.

The docs should also clarify that using NO_GENERATE_PLUGIN_SOURCE means that:

• We must now link to the "backing" target, not the auto-generated plugin target
• The plugin can no longer be be optional