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.

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())