Parses a <usage> tag and adds it the Executable parent instance.
    
    :arg parser: an instance of DocParser to create the DocElement with.

def _parse_usage(tag, parser, parent):
    """Parses a <usage> tag and adds it the Executable parent instance.
    
    :arg parser: an instance of DocParser to create the DocElement with.
    """
    usage = DocElement(tag, parser, parent)
    parent.docstring.append(usage)