note status graph endpoint requirement

server/templates/instance_stats.html.j2

@@ -23,6 +23,11 @@
           <div id="graph-health" class="graph" style="height: 400px; display:none;"></div>
         </div>
       </div>
+      <div class="row">
+        <div class="col">
+          Status graph requires <code>https://{{HOST_DOMAIN}}/.health</code> being accessible.
+        </div>
+      </div>
       <div class="row">
         <div class="col">
           <div id="graph-stats" class="graph" style="height: 400px; display:none;"></div>