Update lease4-add.json

doc/api/lease4-add.json

 {
     "name": "lease4-add",
-    "brief": "The lease4-add command allows for the creation of a new lease.",
+    "brief": "The lease4-add command adds a new IPv4 lease administratively.",
     "description": "See <xref linkend=\"command-lease4-add\"/>",
     "support": [ "kea-dhcp4", "kea-dhcp6" ],
     "avail": "1.3.0",
@@ -11,5 +11,6 @@
         \"ip-address\": \"192.0.2.202\",
         \"hw-address\": \"1a:1b:1c:1d:1e:1f\"
     }
-}"
+}",
+    "cmd-comment": "Note that Kea 1.4 requires an additional argument, subnet-ID, which is optional as of Kea 1.5. A number of other more detailed optional arguments are also supported."
 }