[#998,!588] Updated ha-heartbeat api doc.

doc/sphinx/api/ha-heartbeat.json

 {
     "avail": "1.4.0",
     "brief": [
-        "This command is sent internally by a Kea partner when operating in High-Availability (HA) mode. It retrieves the server's HA state and clock value."
+        "This command is sent internally by a Kea partner when operating in High Availability (HA) mode or by the system administrator to verify the state of the servers with regards to the High Availability. It retrieves the server's HA state and clock value."
     ],
     "cmd-syntax": [
         "{",
         "    \"command\": \"ha-heartbeat\"",
-        "    }"
+        "}"
     ],
     "description": "See <xref linkend=\"ha-server-states\"/>",
     "hook": "high_availability",