MD Settings: Add comment for sync page CSS rule

chrome/browser/resources/settings/people_page/sync_page.html

Index: chrome/browser/resources/settings/people_page/sync_page.html
diff --git a/chrome/browser/resources/settings/people_page/sync_page.html b/chrome/browser/resources/settings/people_page/sync_page.html
index 1e375ac3ff6f88a2d3458469c92d35bcf69f756f..9380e6db990a1eb03bc731627383093b4b0ecb31 100644
--- a/chrome/browser/resources/settings/people_page/sync_page.html
+++ b/chrome/browser/resources/settings/people_page/sync_page.html
@@ -45,6 +45,11 @@
         };
       }
 
+      /**
+       * This is currently necessary because a link inside a disabled
+       * paper-checkbox inherits `pointer-events: none;` and will not work.
+       * See: https://github.com/PolymerElements/paper-checkbox/issues/166
+       */
       #paymentLearnMore {
         pointer-events: initial;
       }