default.settings.php should document how to override strings with context

D7 introduced context for strings and those specific strings can be overridden via settings.php but the docs in settings.php need to illustrate how to do that.

I think the way to change a string with context is additional arrays after the empty one

$conf['locale_custom_strings_en'] = array(
  '' => array(
    'forum'      => 'Discussion board',
    '@count min' => '@count minutes',
  '!long-month-name' => array(
      'May' => 'May', // Need better example that a no-op translation
    ),
  ),
);