Adding note to docs of gtk_widget_set_window()

Adding note to docs of gtk_widget_set_window()

gtk_widget_set_window() does not add a reference to the
owned window... this is an exception to most gtk_foo_set_bar()
functions where as it acts as if it were named gtk_widget_take_window().

Adding note to the docs to avoid people giving the window to the
widget and naturally proceeding to unref the window.