outputHelp(cb) {
    if (!cb) {
      cb = (passthru) => {
        return passthru;
      };
    }
    const cbOutput = cb(this.helpInformation());
    if (typeof cbOutput !== 'string' && !Buffer.isBuffer(cbOutput)) {
      throw new Error('outputHelp callback must return a string or a Buffer');
    }
    process.stdout.write(cbOutput);
    this.emit(this._helpLongFlag);
  }
Output help information for this command.

When listener(s) are available for the helpLongFlag
those callbacks are invoked.

@api public