Update FLAG_ONLY_ALERT_ONCE docs

Update FLAG_ONLY_ALERT_ONCE docs to match implementation
from Android P and onwards.

Bug: 222328375
Test: atest BuzzBeepBlinkTest
Change-Id: Ic058b8265551362b04b9cd0363e43a4c08ecd736

core/java/android/app/Notification.java

@@ -622,6 +622,9 @@ public class Notification implements Parcelable
      * Bit to be bitwise-ored into the {@link #flags} field that should be
      * set if you would only like the sound, vibrate and ticker to be played
      * if the notification was not already showing.
+     *
+     * Note that using this flag will stop any ongoing alerting behaviour such
+     * as sound, vibration or blinking notification LED.
      */
     public static final int FLAG_ONLY_ALERT_ONCE    = 0x00000008;
 
@@ -4601,6 +4604,9 @@ public class Notification implements Parcelable
          * Set this flag if you would only like the sound, vibrate
          * and ticker to be played if the notification is not already showing.
          *
+         * Note that using this flag will stop any ongoing alerting behaviour such
+         * as sound, vibration or blinking notification LED.
+         *
          * @see Notification#FLAG_ONLY_ALERT_ONCE
          */
         @NonNull