update reference doc

REFERENCE.md

@@ -4177,12 +4177,19 @@ Default value: `undef`
 
 Data type: `Optional[Variant[String, Array]]`
 
-Where to write error log. May add additional options like error level to
-the end. May set to 'absent', in which case it will be omitted in this
+Where to write error log. May set to 'absent', in which case it will be omitted in this
 server stanza (and default to nginx.conf setting)
 
 Default value: `undef`
 
+##### <a name="-nginx--resource--server--error_log_severity"></a>`error_log_severity`
+
+Data type: `Optional[Nginx::ErrorLogSeverity]`
+
+Optional error level. Defaults to nginx.conf setting
+
+Default value: `$nginx::nginx_error_log_severity`
+
 ##### <a name="-nginx--resource--server--passenger_cgi_param"></a>`passenger_cgi_param`
 
 Data type: `Optional[Hash]`