Editorial: Clarify that PlainTime ignores the [[Days]] of its record

We have similar language in ISO Date-Time Record.

Author: ptomato

spec/plaintime.html

@@ -383,7 +383,7 @@ <h1>Properties of Temporal.PlainTime Instances</h1>
             [[Time]]
           </td>
           <td>
-            A Time Record.
+            A Time Record. The [[Days]] field is ignored.
           </td>
         </tr>
       </table>