Add visual editor to Media Edit Screen

This function added to functions.php does exactly that:

// Enables Editor in Media edit screen.
function add_editor_support_for_attachments($settings) {
    if( get_post_type() == 'attachment'){
      $quicktags_settings = array( 'buttons' => 'strong,em,link,block,del,ins,ul,ol,li,code,close' );
      $settings = array(
        'wpautop'       => true,
        'textarea_name' => 'content',
        'textarea_rows' => 10,
        'media_buttons' => false,
        'tinymce' => true,
        'quicktags' => $quicktags_settings,
      );
      return $settings;
   }
}
add_filter( 'wp_editor_settings', 'add_editor_support_for_attachments' );