docs(auth_http): Update config doc

apps/emqx_auth_http/etc/emqx_auth_http.conf

@@ -73,6 +73,7 @@ auth.http.super_req.headers.content-type = application/x-www-form-urlencoded
 auth.http.super_req.params = clientid=%c,username=%u
 
 ## HTTP URL API path for ACL Request
+## Comment out this config to disable ACL checks
 ##
 ## Value: URL
 ##

apps/emqx_auth_http/priv/emqx_auth_http.schema

@@ -60,6 +60,10 @@ end}.
   end
 end}.
 
+%% @doc URL for ACL checks. Example: http://127.0.0.1:80/mqtt/acl
+%% ACL checks are disabled for this plugin if this config is
+%% commented out from the config file, or when the overriding
+%% environment variable is set to empty string.
 {mapping, "auth.http.acl_req.url", "emqx_auth_http.acl_req", [
   {datatype, string}
 ]}.