[EoC] Update UKM TRIGGER_EVENT description

Update the TRIGGER_EVENT description in ukm.xml to accurately reflect
the values recorded. This update was missed in previous patchset that
added metrics for the toolbar button triggering UI.

BUG=859563

Change-Id: Ia270c452195a46120802d6de00032583007f7459
Reviewed-on: https://chromium-review.googlesource.com/c/1333998Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608999}

tools/metrics/ukm/ukm.xml

@@ -1226,10 +1226,11 @@ be describing additional metrics about the same event.
   </metric>
   <metric name="TriggerEvent">
     <summary>
-      Emits an integer in the range 0-1 to indicate whether the peeking sheet
-      was ever triggered and how it was triggered. A value of 0 indicates
-      never-shown, a 1 indicates peeked in response to a reverse-scroll (the
-      only trigger currently supported).
+      Emits an integer in the range 0-2 to indicate whether the contextual
+      suggestions UI was ever triggered and how it was triggered. A value of 0
+      indicates never-shown, a 1 indicates peeked in response to a
+      reverse-scroll (deprecated), and 2 indicates that the toolbar button was
+      shown (the only trigger currently supported).
     </summary>
   </metric>
 </event>