Flush: Add javadoc for exception when no active stream.

PiperOrigin-RevId: 579225491

libraries/common/src/main/java/androidx/media3/common/VideoFrameProcessor.java

@@ -333,6 +333,8 @@ public interface VideoFrameProcessor {
    *
    * @throws UnsupportedOperationException If the {@code VideoFrameProcessor} does not accept
    *     {@linkplain #INPUT_TYPE_SURFACE surface input}.
+   * @throws IllegalStateException If {@link #registerInputStream} is not called before calling this
+   *     method.
    */
   void flush();