Improve javadoc of Window.windowStartTimeMs

PiperOrigin-RevId: 346346359

library/common/src/main/java/com/google/android/exoplayer2/Timeline.java

@@ -166,7 +166,7 @@ public abstract class Timeline {
 
 
     /**
     /**
      * The window's start time in milliseconds since the Unix epoch, or {@link C#TIME_UNSET} if
      * The window's start time in milliseconds since the Unix epoch, or {@link C#TIME_UNSET} if
-     * unknown or not applicable. For informational purposes only.
+     * unknown or not applicable.
      */
      */
     public long windowStartTimeMs;
     public long windowStartTimeMs;