Revise the description of ExternalStorageReadOnly to make it clearer.

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 5796 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': '''This policy is effective only when ExternalStorageDisabled is set to false or not configured.
+      'desc': '''When this policy is set to true, users cannot write anything to external storage devices.
 
-      When this policy is set to true, users cannot write anything to external storage devices.
+      If this setting is set to false or not configured, then users can create and modify files of external storage devices which are physically writable.
 
-      If this setting is disabled or not configured then users can create and modify files of external storage devices which are physically writable.
+      The ExternalStorageDisabled policy takes precedence over this policy - if ExternalStorageDisabled is set to true, then all access to external storage is disabled and this policy is consequently ignored.
 
       Dynamic refresh of this policy is supported in M56 and later.''',
       'features': {
         'dynamic_refresh': True,
         'per_profile': True,
       },
       'example_value': True
     },
     {
       'name': 'AudioOutputAllowed',
@@ skipping 3568 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': [],
 }