docs: def _merge_doc(original, to_merge):
      # type: (str, str) -> str
      """Merge two usage strings together.
  
      Args:
          original: The source of headers and initial section lines.
          to_merge: The source for the additional section lines to append.
  
      Returns:
          A new usage string that contains information from both usage strings.
      """
      if not original:
          return to_merge or ''
      if not to_merge:
          return original or ''
      sections = []
      for name in ('usage', 'arguments', 'options'):
          sections.append(_merge_section(
              _get_section(name, original),
              _get_section(name, to_merge)
          ))
      return format_usage('\n\n'.join(s for s in sections).rstrip())