Correct AllowCrossOriginAuthPrompt policy description

This policy controls the ability of cross-site images to trigger auth
prompts when a page is loaded. The policy applies only to images, and
applies to all HTTP authentication types (Basic/Digest/NTLM/Negotiate).
This CL updates the policy description for accuracy.

Bug: 1124044
Change-Id: I82f53e5ca644d39506f94e91a2a0442a5682069a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390857Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Eric Lawrence [MSFT] <ericlaw@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#804362}

components/policy/resources/policy_templates.json

@@ -4327,9 +4327,9 @@
       'id': 89,
       'caption': '''Cross-origin HTTP Basic Auth prompts''',
       'tags': ['website-sharing'],
-      'desc': '''Setting the policy to Enabled allows third-party subcontent on a page to open an HTTP Basic Auth pop-up.
+      'desc': '''Setting the policy to Enabled allows third-party images on a page to show an authentication prompt.
 
-       Setting the policy to Disabled or leaving it unset renders third-party subcontent unable to open an HTTP Basic Auth pop-up.
+       Setting the policy to Disabled or leaving it unset renders third-party images unable to show an authentication prompt.
 
       Typically, this policy is Disabled as a phishing defense.''',
     },