Readd text to AllowPopupsDuringPageUnload that was accidentally removed

This text was incorrectly removed in https://chromium-review.googlesource.com/c/chromium/src/+/2217873


Bug: 1018157
Change-Id: Id78aa3cf5012076780bc678c5106aa2a112284e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302830Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789475}

components/policy/resources/policy_templates.json

@@ -18834,7 +18834,11 @@
       'tags': [],
       'desc': '''Setting the policy to True allows pages to show pop-ups while the pages unload.
 
-      Setting the policy to False or leaving it unset prevents pages from showing pop-ups while the pages unload.''',
+      Setting the policy to False or leaving it unset prevents pages from showing pop-ups while the pages unload.
+
+      This policy will be removed in Chrome 88.
+
+      See https://www.chromestatus.com/feature/5989473649164288.''',
     },
     {
       'name': 'DeviceWilcoDtcConfiguration',