code: public ApnsPayloadBuilder setAlertSubtitle(final String alertSubtitle) {
          this.alertSubtitle = alertSubtitle;
  
          this.localizedAlertSubtitleKey = null;
          this.localizedAlertSubtitleArguments = null;
  
          return this;
      }
docs: <p>Sets a subtitle for the notification. Clears any previously-set localized subtitle key and arguments.</p>
  
  <p>By default, no subtitle is included. Requires iOS 10 or newer.</p>
  
  @param alertSubtitle the subtitle for this push notification
  
  @return a reference to this payload builder
  
  @since 0.8.1