Declare dynamic refresh support for ExternalStorageReadOnly 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 5794 matching lines @@
       If this setting is disabled or not configured then users can use all supported types of external storage on their device.''',
     },
     {
       'name': 'ExternalStorageReadOnly',
       'id': 343,
       'type': 'main',
       'schema': { 'type': 'boolean' },
       'tags': [],
       'supported_on': ['chrome_os:54-'],
       'caption': '''Treat external storage devices as read-only.''',
-      'desc': '''When this policy is set to true, users cannot write anything to external storage devices.''',
+      'desc': '''Treat external storage devices as read-only.
+
+      This policy is effective only when ExternalStorageDisabled is set to false or not configured.
+
+      When this policy is set to true, users cannot write anything to external storage devices.
+
+      If this setting is disabled or not configured then users can create and modify files of external storage devices which are physically writable.
+
+      Dynamic refresh of this policy is supported in M56 and later.''',
       'features': {
-        'dynamic_refresh': False,
+        'dynamic_refresh': True,
         'per_profile': True,
       },
       'example_value': True
     },
     {
       'name': 'AudioOutputAllowed',
       'type': 'main',
       'schema': { 'type': 'boolean' },
       'supported_on': ['chrome_os:23-'],
       'features': {
@@ skipping 3562 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': [],
 }