Retrieve description for a sidebar.

When registering sidebars a ‘description’ parameter can be included that
describes the sidebar for display on the widget administration panel.

function wp_sidebar_description( $id ) {
	if ( ! is_scalar( $id ) ) {
		return;
	}

	global $wp_registered_sidebars;

	if ( isset( $wp_registered_sidebars[ $id ]['description'] ) ) {
		return wp_kses( $wp_registered_sidebars[ $id ]['description'], 'sidebar_description' );
	}
}