docs: def validate_env(app):
      """Purge expired values from the environment.
  
      When certain configuration values change, related values in the
      environment must be cleared. While Sphinx can rebuild documents on
      configuration changes, it does not notify extensions when this
      happens. Instead, cache relevant values in the environment in order
      to detect when they change.
  
      Args:
          app: The Sphinx application.
      """
  
      if not hasattr(app.env, 'javalink_config_cache'):
          app.env.javalink_config_cache = {}
  
      for conf_attr, (_, _, env_attr) in ref.CONFIG_VALUES.iteritems():
          if not env_attr:
              continue
  
          value = getattr(app.config, conf_attr)
          cached = app.env.javalink_config_cache.get(conf_attr, value)
  
          app.env.javalink_config_cache[conf_attr] = value
          if value != cached:
              app.verbose('[javalink] config.%s has changed, clearing related env', conf_attr)
              delattr(app.env, env_attr)