Clarify docs on ExoPlayer.setVideoEffects() re calling prepare()

The previous wording suggested that `setVideoEffects()` may **only** be
called before `prepare()`, i.e. the effect cannot be changed during
playback. The intent is instead that `setVideoEffects()` must be called
once before playback in order to configure the effects pipeline, but
the effect can then be changed during playback by further calls to
`setVideoEffects()`.

Issue: androidx/media#1393
PiperOrigin-RevId: 641853629

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/ExoPlayer.java

@@ -1691,7 +1691,9 @@ public interface ExoPlayer extends Player {
    *   <li>This feature does not work with {@linkplain Effect effects} that update the frame
    *       timestamps.
    *   <li>This feature does not work with DRM-protected content.
-   *   <li>This method should be called before calling {@link #prepare()}.
+   *   <li>This method must be called at least once before calling {@link #prepare()} (in order to
+   *       set up the effects pipeline). The effects can be changed during playback by subsequent
+   *       calls to this method after {@link #prepare()}.
    * </ul>
    *
    * @param videoEffects The {@link List} of {@linkplain Effect video effects} to apply.