Revise the description of ExternalStorageReadOnly to make it clearer.

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 3ed6ea907975b6d9659f519b00a1aea31c2f3b58..63263602d27e3e56fbaf0d15fd53dc6ace77ec5f 100644
--- a/components/policy/resources/policy_templates.json
+++ b/components/policy/resources/policy_templates.json
@@ -5814,11 +5814,11 @@
       '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': {