chore: add missing parameter in available variables

apps/emqx_auth_http/etc/emqx_auth_http.conf

@@ -96,6 +96,7 @@ auth.http.acl_req.headers.content-type = application/x-www-form-urlencoded
 ## When the request method is POST, the final format is determined by content-type
 ##
 ## Available Variables:
+##  - %A: access (1 - subscribe, 2 - publish)
 ##  - %u: username
 ##  - %c: clientid
 ##  - %a: ipaddress
@@ -104,6 +105,7 @@ auth.http.acl_req.headers.content-type = application/x-www-form-urlencoded
 ##  - %p: sockport of server accepted
 ##  - %C: common name of client TLS cert
 ##  - %d: subject of client TLS cert
+##  - %t: topic
 ##
 ## Value: <K1>=<V1>,<K2>=<V2>,...
 auth.http.acl_req.params = access=%A,username=%u,clientid=%c,ipaddr=%a,topic=%t,mountpoint=%m