description(str, argsDescription) {
    if (str === undefined && argsDescription === undefined)
      return this._description;
    this._description = str;
    if (argsDescription) {
      this._argsDescription = argsDescription;
    }
    return this;
  }
Set the description.

@param {string} [str]
@param {object} [argsDescription]
@return {(string|Command)}