Fix docs for FrameMetrics.TOTAL_DURATION

The existing docs are incorrect and misleading. This change
fixes those docs to be more accurate.

Bug: 244193434
Test: Doc-only fix, no testing needed
Change-Id: I454786c2ef888417e96d7139efd1c922537188d6

core/java/android/view/FrameMetrics.java

@@ -116,8 +116,10 @@ public final class FrameMetrics {
      * and be issued to the display subsystem.
      * </p>
      * <p>
-     * Equal to the sum of the values of all other time-valued metric
-     * identifiers.
+     * The total duration is the difference in time between when the frame
+     * began and when it ended. This value may not be exactly equal to the
+     * sum of the values of all other time-valued metric identifiers because
+     * some stages may happen concurrently.
      * </p>
      */
     public static final int TOTAL_DURATION = 8;