Add documentation warning about chrome://* in URLBlacklist policy.

components/policy/resources/policy_templates.json

 {
 # policy_templates.json - Metafile for policy templates
 #
 # The content of this file is evaluated as a Python expression.
 #
 # This file is used as input to generate the following policy templates:
 # ADM, ADMX+ADML, MCX/plist and html documentation.
 #
 # Policy templates are user interface definitions or documents about the
 # policies that can be used to configure Chrome. Each policy is a name-value
@@ skipping 4168 matching lines @@
       Each URL pattern can either be a pattern for local files or a generic URL pattern. Local file patterns are of the format 'file://path', where path should be an absolute path to block. All file system locations for which that path is a prefix will be blocked.
 
       A generic URL pattern has the format 'scheme://host:port/path'.
       If present, only the specified scheme will be blocked. If the scheme:// prefix is not specified, all schemes are blocked.
       The host is required and can be a hostname or an IP address. Subdomains of a hostname will also be blocked. To prevent blocking subdomains, include a '.' before the hostname. The special hostname '*' will block all domains.
       The optional port is a valid port number from 1 to 65535. If none is specified, all ports are blocked.
       If the optional path is specified, only paths with that prefix will be blocked.
 
       Exceptions can be defined in the URL whitelist policy. These policies are limited to 1000 entries; subsequent entries will be ignored.
 
+      Note that it is not recommended to block internal 'chrome://*' URLs since this may lead to unexpected errors.
+
       If this policy is not set no URL will be blacklisted in the browser.''',
     },
     {
       'name': 'URLWhitelist',
       'type': 'list',
       'schema': {
         'type': 'array',
         'items': { 'type': 'string' },
       },
       'supported_on': [
@@ skipping 3977 matching lines @@
       'desc': '''Text appended in parentheses next to the policies top-level container to indicate that those policies are of the Recommended level''',
       'text': 'Default Settings (users can override)',
     },
     'doc_complex_policies_on_windows': {
       'desc': '''Text pointing the user to a help article for complex policies on Windows''',
       'text': '''encoded as a JSON string, for details see <ph name="COMPLEX_POLICIES_URL">https://www.chromium.org/administrators/complex-policies-on-windows<ex>https://www.chromium.org/administrators/complex-policies-on-windows</ex></ph>''',
     },
   },
   'placeholders': [],
 }