Update ha-heartbeat.json

doc/api/ha-heartbeat.json

@@ -7,6 +7,8 @@
     "support": [ "kea-dhcp4", "kea-dhcp6" ],
     "avail": "1.4.0",
     "hook": "high_availability",
-    "cmd-syntax": "tbd",
-    "resp-syntax": "tbd"
+    "cmd-syntax": "{
+    \"command\": \"ha-heartbeat\"
+,
+    "resp-syntax": The response to this command is different from the typical command response. The response will include server state (see <xref linkend=\"ha-server-states\"/>" plus the current clock value. 
 }