Document widget base class filters and actions

[theenoahmason]

do_action( "wp_backstage_widget_output_{$this->slug}", $args, $instance );

do_action( "wp_backstage_widget_form_{$this->slug}", $instance, $this->id_base, $this->number );

return apply_filters( "wp_backstage_widget_save_{$this->slug}", $new_instance, $old_instance );