Added overview on DDL+mixed versions

Signed-off-by: Dj Walker-Morgan <dj.walker-morgan@enterprisedb.com>

Author: djw-m

product_docs/docs/pgd/5.8/ddl/ddl-overview.mdx

@@ -75,3 +75,10 @@ When using the [`bdr.replicate_ddl_command`](/pgd/latest/reference/functions#bdr
 parameter directly with the third argument, using the specified
 [`bdr.ddl_locking`](/pgd/latest/reference/pgd-settings#bdrddl_locking) setting only for the DDL commands passed to that
 function.
+
+## DDL and mixed PostgreSQL versions
+
+PGD does not support DDL replication between different major Postgres versions in a cluster. Most of the time, this is not an issue because clusters will be running the same major version of Postgres. This is not the case though when performing a rolling upgrade of a cluster from one major version to another. In this case, DDL replication is not supported until all nodes have been upgraded to the same major version and should not be used.
+
+Special care should be taken in the upgrade process when updating extensions as the scripts may trigger DDL replication. In this case, if the scripts must be run before upgrading is complete, `bdr.ddl_replication` setting should be set to `off` while running the script. See the [Rolling upgrade guide](/pgd/latest/upgrade/rolling-upgrade) for more information on how to perform a rolling upgrade of a PGD cluster.
+