UKM: clarify documentation around Web Audio Audible Time.

Bug: None
Change-Id: I99172fb7a36e002fe535c2176189368cee3283e1
Reviewed-on: https://chromium-review.googlesource.com/c/1357384Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613357}

tools/metrics/ukm/ukm.xml

@@ -2073,8 +2073,7 @@ be describing additional metrics about the same event.
   </summary>
   <metric name="AudibleTime">
     <summary>
-      The number of seconds of audio that were audible during the entire
-      lifetime of the AudioContext.
+      Audible time in milliseconds for this event.
     </summary>
   </metric>
   <metric name="IsMainFrame">