Add the default language text to dartdoc in `LocaleKeys`

[homeserve-lsaudon]

Hi,

Thanks for this package.

It would help me find the key to use more easily.

ex:

abstract class LocaleKeys {
  /// You not have money
  static const money_zero = 'money.zero';
  /// You have {} dollar
  static const money_one = 'money.one';
}