DBZ-619 Document wal2json on RDS limitations

docs/connectors/postgresql.asciidoc

@@ -53,6 +53,13 @@ It is possible to monitor PostgreSQL database running in https://aws.amazon.com/
 * Set `plugin.name` Debezium parameter to `wal2json`. Note that RDS is running older version of the plug-in. We are trying to detect that it is the case but the detection is not 100 % reliable. In case of problems use `wal2json_rds` value to manually override the detection routine.
 * Use database master account for replication as RDS currently does not support setting of `REPLICATION` privilege for another account.
 
+==== Known limitations
+The `wal2json` plug-in used in Amazon RDS environment is not fully up-to-date with the recent additions to the upstream project.
+Till the plug-in is updated please expect certain limits in capabilities vs the plug-in version used by Debezium.
+
+* The replication message does not carry information about type contraints like length or scale or `NULL`/`NOT NULL`.
+Debezium is thus not able to detect changes in type constraints (https://issues.jboss.org/browse/DBZ-504[DBZ-504]).
+
 [[output-plugin]]
 === Installing the logical decoding output plugin