Add a caveat to URLBlacklist's policy description

Bug: 896225
Change-Id: I35a7355a3d3b5fb9ad8d5425e2855985ab36e63a
Reviewed-on: https://chromium-review.googlesource.com/c/1370824
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616002}

components/policy/resources/policy_templates.json

@@ -5596,6 +5596,8 @@
 
       Note that it is not recommended to block internal 'chrome://*' URLs since this may lead to unexpected errors.
 
+      Note that this policy does not prevent the page updating dynamically through JavaScript. For example, if you block 'example.com/abc', users might still be able to visit 'example.com' and click on a link to visit 'example.com/abc', as long as the page does not refresh.
+
       If this policy is not set no URL will be blacklisted in the browser.''',
       'arc_support': 'Android apps may voluntarily choose to honor this list. You cannot force them to honor it.',
     },