ovn-sb: Describe enum error for Service_Monitor.status.

the ovn-sb.ovsschema Service_Monitor.status column has an enum that
lists three possible values - online,offline, and error. Even though
nothing sets the error enum it should still be listed as a possibility in
documentation.

Fixes: ba0d6ea ("ovn-northd: Add support for Load Balancer health check")
Assisted-by: Claude Opus 4.6, Claude Code
Signed-off-by: Jacob Tanenbaum <jtanenba@redhat.com>
Signed-off-by: Mark Michelson <mmichels@redhat.com>

Author: JacobTanenbaum

ovn-sb.xml

@@ -5476,6 +5476,11 @@ tcp.flags = RST;
           <code>offline</code>.
        </p>
 
+        <p>
+          The value <code>error</code> is also valid but is not
+          currently set by <code>ovn-controller</code>.
+        </p>
+
       </column>
     </group>