PGD: update description for "bdr.join_node_group()"

From PGD 5.6, database structure will never be synchronized to
a witness server, which represents a minor behaviour change compared
to previous versions, and means the "synchronize_structure" parameter,
if explicitly set to "all", will be forcibly overridden (a NOTICE will
be emitted noting that).

DOCS-1029.

product_docs/docs/pgd/5.6/reference/nodes-management-interfaces.mdx

@@ -291,11 +291,12 @@ bdr.join_node_group (
      node.
 -   `wait_for_completion` — Wait for the join process to complete before
      returning. Defaults to `true`.
--   `synchronize_structure` — Set the kind of structure (schema) synchronization
-     to do during the join.  The default setting is `all`, which synchronizes
-     the complete database structure, The other available setting is `none`, which doesn't
-     synchronize any structure. However, it still synchronizes data (except for witness
-     nodes, which by design don't synchronize data).
+-   `synchronize_structure` — Specifies whether to perform database structure (schema)
+     synchronization during the join. `all`, the default setting, synchronizes
+     the complete database structure. `none` does not synchronize any structure.
+     However, data will still be synchronized, meaning the database structure must already
+     be present on the joining node. Note that by design, neither schema nor data will
+     ever be synchronized to witness nodes.
 -   `pause_in_standby` — Optionally tells the join process to join only as a
      logical standby node, which can be later promoted to a full member.
      This option is deprecated and will be disabled or removed in future