'Make usage strings late so that plugins can be initialised late
:param plugins: all discovered plugins
:param help_arg: False for none; True for --help; "TOPIC" for --help TOPIC
:rtype: str
:returns: a short usage string for the top of --help TOPIC)'

def _usage_string(self, plugins, help_arg):

    if ('nginx' in plugins):
        nginx_doc = '--nginx                                            Use    the    Nginx    plugin    for    authentication    &    installation'
    else:
        nginx_doc = '(the    certbot    nginx    plugin    is    not    installed)'
    if ('apache' in plugins):
        apache_doc = '--apache                                        Use    the    Apache    plugin    for    authentication    &    installation'
    else:
        apache_doc = '(the    certbot    apache    plugin    is    not    installed)'
    usage = SHORT_USAGE
    if (help_arg == True):
        print(((usage + (COMMAND_OVERVIEW % (apache_doc, nginx_doc))) + HELP_USAGE))
        sys.exit(0)
    elif (help_arg in self.COMMANDS_TOPICS):
        print((usage + self._list_subcommands()))
        sys.exit(0)
    elif (help_arg == 'all'):
        usage += (COMMAND_OVERVIEW % (apache_doc, nginx_doc))
    else:
        custom = VERB_HELP_MAP.get(help_arg, {}).get('usage', None)
        usage = (custom if custom else usage)
    return usage
