Doc: Add "Attach Partition" limitation during logical replication.

ATTACHing a table into a partition tree whose root is published using a
publication with publish_via_partition_root set to true does not result in
the table's existing contents being replicated. This happens because
subscriber doesn't consider replicating the newly attached partition as
the root table is already in a 'ready' state.

This behavior was introduced in PG13 (83fd4532) where we allowed to
publish partition changes via ancestors.

We can consider fixing this limitation in the future.

Author: Amit Langote
Reviewed-by: Hou Zhijie, Amit Kapila
Backpatch-through: 13
Discussion: https://postgr.es/m/OS0PR01MB5716E97F00732B52DC2BBC2594989@OS0PR01MB5716.jpnprd01.prod.outlook.com

doc/src/sgml/ref/create_publication.sgml

@@ -189,6 +189,13 @@ CREATE PUBLICATION <replaceable class="parameter">name</replaceable>
    <command>UPDATE</command>, or it may not be published at all.
   </para>
 
+  <para>
+   <command>ATTACH</command>ing a table into a partition tree whose root is
+   published using a publication with <literal>publish_via_partition_root</literal>
+   set to <literal>true</literal> does not result in the table's existing contents
+   being replicated.
+  </para>
+
   <para>
    <command>COPY ... FROM</command> commands are published
    as <command>INSERT</command> operations.