message: docs(helpers): better describe vars
diff: @@ -36,20 +36,20 @@ function noop () { /* do nothing */ }
  /**
  * key: {function} user's callback function
  - * values: {Map} listeners, key: msg, values: generated or user's callback functions
  + * values: {Map} listeners, key: message type, values: generated or user's callback functions
  */
  const messageListeners: Map<browser.runtime.onMessageEvent, Map<string, browser.runtime.onMessageEvent>> = new Map()
  /**
  * For self page messaging
  * key: {function} user's callback function
  - * values: {Map} listeners, key: msg, values: generated or user's callback functions
  + * values: {Map} listeners, key: message type, values: generated or user's callback functions
  */
  const messageSelfListeners: Map<browser.runtime.onMessageEvent, Map<string, browser.runtime.onMessageEvent>> = new Map()
  /**
  * key: {function} user's callback function
  - * values: {Map} listeners, key: msg, values: generated or user's callback functions
  + * values: {Map} listeners, key: message type, values: generated or user's callback functions
  */
  const storageListeners: Map<StorageListenerCb, Map<string, StorageListenerCb>> = new Map()
  