def _get_docstring_type_name(var_doc: str) -> str:
    """
    Get the string of argument or return value type's description
    from docstring.

    Parameters
    ----------
    var_doc : str
        Docstring's part of argument or return value.

    Returns
    -------
    type_name : str
        Argument or return value's type description.
    """
    type_name: str = var_doc.split('\n')[0]
    colon_exists: bool = ':' in type_name
    if not colon_exists:
        return ''
    type_name = type_name.split(':')[1]
    type_name = type_name.split(',')[0]
    type_name = type_name.strip()
    return type_name