docs: def _is_definition_section(source):
      """Determine if the source is a definition section.
  
      Args:
          source: The usage string source that may be a section.
  
      Returns:
          True if the source describes a definition section; otherwise, False.
      """
      try:
          definitions = textwrap.dedent(source).split('\n', 1)[1].splitlines()
          return all(
              re.match(r'\s\s+((?!\s\s).+)\s\s+.+', s) for s in definitions)
      except IndexError:
          return False