Source
File: wp-includes/customize/class-wp-widget-area-customize-control.php
public function render_content() {
$id = 'reorder-widgets-desc-' . str_replace( array( '[', ']' ), array( '-', '' ), $this->id );
?>
<button type="button" class="button add-new-widget" aria-expanded="false" aria-controls="available-widgets">
<?php _e( 'Add a Widget' ); ?>
</button>
<button type="button" class="button-link reorder-toggle" aria-label="<?php esc_attr_e( 'Reorder widgets' ); ?>" aria-describedby="<?php echo esc_attr( $id ); ?>">
<span class="reorder"><?php _e( 'Reorder' ); ?></span>
<span class="reorder-done"><?php _e( 'Done' ); ?></span>
</button>
<p class="screen-reader-text" id="<?php echo esc_attr( $id ); ?>"><?php _e( 'When in reorder mode, additional controls to reorder widgets will be available in the widgets list above.' ); ?></p>
<?php
}