docs: def truth(message, expected=None):
      """ Convenience decorator that applies [`Check`](#check) to a callable.
  
      ```python
      from good import truth
  
      @truth(u'Must be an existing directory')
      def isDir(v):
          return os.path.isdir(v)
      ```
  
      :param message: Validation error message
      :type message: unicode
      :param expected: Expected value string representation, or `None` to get it from the wrapped callable
      :type expected: None|str|unicode
      :return: decorator
      :rtype: callable
      """
      def decorator(func):
          return update_wrapper(Check(func, message, expected), func)
      return decorator