Add note about how large block window for maintenance can be

Author: hmusum

en/reference/applications/deployment.html

@@ -185,7 +185,9 @@ <h2 id="block-change">block-change</h2>
   <tr>
     <td>maintenance
     <td>No, default <code>false</code>
-    <td>Set to <code>true</code> to disallow Vespa maintenance operations. This is best effort, maintenance can still happen (e.g. for security reasons).
+    <td>Set to <code>true</code> to disallow Vespa maintenance operations. This is best effort, maintenance
+      can still happen (e.g. for security reasons). The block window for maintenance should be open at least
+      10% of the time calculated over a week, that is, at least 17 hours per week.
   </tr>
   <tr>
     <td>days