Add warning to policy docs about disabling extensions using devtools.

components/policy/resources/policy_templates.json

Index: components/policy/resources/policy_templates.json
diff --git a/components/policy/resources/policy_templates.json b/components/policy/resources/policy_templates.json
index 81471dce1ed108488d47d0707afe8b8a072e3e72..da5c833869c321967f4febe29528a71ba72eb039 100644
--- a/components/policy/resources/policy_templates.json
+++ b/components/policy/resources/policy_templates.json
@@ -1840,7 +1840,11 @@
 
           Users will be unable to uninstall extensions that are specified by this policy. If you remove an extension from this list, then it will be automatically uninstalled by <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph>. Extensions specified in this list are also automatically whitelisted for installation; the ExtensionsInstallBlacklist does not affect them.
 
-          If this policy is left not set the user can uninstall any extension  in <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph>.''',
+          Note that the source code of any extension may be altered (potentially
+          rendering the extension dysfunctional) by using Developer Tools. If
+          this is a concern, the DeveloperToolsDisabled policy should be set.
+
+          If this policy is left not set the user can uninstall any extension in <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph>.''',
           'label': '''Extension IDs and update URLs to be silently installed''',
         },
         {