add about section for bad hosts

server/templates/about.html.j2

@@ -27,7 +27,7 @@
 
     <p>Explanations</p>
     <ul>
-      <li><code>Healthy</code> stands for hosts which are reachable and pass a content check.</li>
+      <li><code>Healthy</code> stands for hosts which are reachable and pass a content check. Known bad hosts are marked with a ❓, these instances block healthchecks.</li>
       <li><code>Response Time</code> is the response time over the last {{ping_avg_interval_h}} hours. This is not a network ping.</li>
       <li><code>Average</code> is the time average over the last {{ping_avg_interval_h}} hours.</li>
       <li><code>RSS</code> whether the host has RSS feeds enabled.</li>