cli: 'id' option for 'nmcli con add type vlan' is mandatory (rh #953397)

So update documentation accordingly.

https://bugzilla.redhat.com/show_bug.cgi?id=953397

cli/src/connections.c

@@ -305,7 +305,7 @@ usage_connection_add (void)
 	         "    bluetooth:    [addr <bluetooth address>]\n"
 	         "                  [bt-type panu|dun-gsm|dun-cdma]\n\n"
 	         "    vlan:         dev <parent device (connection  UUID, ifname, or MAC)>\n"
-	         "                  [id <VLAN id>]\n"
+	         "                  id <VLAN ID>\n"
 	         "                  [flags <VLAN flags>]\n"
 	         "                  [ingress <ingress priority mapping>]\n"
 	         "                  [egress <egress priority mapping>]\n"

man/nmcli.1.in

@@ -454,8 +454,8 @@ Note: use quotes around \fB*\fP to suppress shell expansion.
 .B vlan:
 .IP "\fIdev <parent device (connection UUID, ifname, or MAC)>\fP" 42
 \(en parent device this VLAN is on
-.IP "\fI[id <VLAN id>]\fP" 42
-\(en VLAN ID
+.IP "\fIid <VLAN ID>\fP" 42
+\(en VLAN ID in range <0-4095>
 .IP "\fI[flags <VLAN flags>]\fP" 42
 \(en flags
 .IP "\fI[ingress <ingress priority mapping>]\fP" 42