SynthesisCallback documentation update

Clarification about when android.speech.tts.SynthesisCallback.done()
should be called.

Bug: 2481825
Change-Id: Ic781a6facc2d9acb3f06afb952fbac0b494c56cf

Author: pszc

core/java/android/speech/tts/SynthesisCallback.java

@@ -22,10 +22,11 @@
  * {@link #start}, then {@link #audioAvailable} until all audio has been provided, then finally
  * {@link #done}.
  *
- *
  * {@link #error} can be called at any stage in the synthesis process to
  * indicate that an error has occurred, but if the call is made after a call
  * to {@link #done}, it might be discarded.
+ *
+ * After {@link #start} been called, {@link #done} must be called regardless of errors.
  */
 public interface SynthesisCallback {
     /**
@@ -72,6 +73,8 @@ public interface SynthesisCallback {
      * This method should only be called on the synthesis thread,
      * while in {@link TextToSpeechService#onSynthesizeText}.
      *
+     * This method has to be called if {@link #start} was called.
+     *
      * @return {@link TextToSpeech#SUCCESS} or {@link TextToSpeech#ERROR}.
      */
     public int done();