[docs] explain how to attach popover to custom widgets in Gtk

The gtk4 migration guide states:

If you want to make a custom widget that has an attached popover, you need to call gtk_popover_present() in your Gtk.WidgetClass.size_allocate vfunc, in order to update the positioning of the popover.

However, this isn't mentioned in the documentation for Gtk.Popover (or Gtk.PopoverMenu). The overview document for Popover should probably mention how to use it with a custom widget, and maybe include an example.