cros: Update SystemUse24HourClock policy description

To reflect actual implementation

Bug: none
Change-Id: I7cdb669ffaa6aacf0b1442db49026aa8d8e26ff4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414034Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807816}

components/policy/resources/policy_templates.json

@@ -10386,9 +10386,13 @@
       'id': 236,
       'caption': '''Use 24 hour clock by default''',
       'tags': [],
-      'desc': '''Setting the policy to True or leaving it unset gives a device's sign-in screen a 24-hour clock format. User sessions also default to this format, but users can change an account's clock format.
+      'desc': '''Setting the policy to True gives a device's sign-in screen a 24-hour clock format.
 
-      Setting the policy to False gives a device the 12-hour clock format.''',
+      Setting the policy to False gives a device's sign-in screen a 12-hour clock format.
+
+      Leaving the policy unset makes a device use the format from the current locale.
+
+      User sessions also default to the device format, but users can change an account's clock format.''',
     },
     {
       'name': 'TouchVirtualKeyboardEnabled',