Doc: Update documentation on partitioning vs. foreign tables.

The limitations that it is not allowed to create/attach a foreign table
as a partition of an indexed partitioned table were not documented.

Reported-By: Stepan Yankevych
Author: Etsuro Fujita
Reviewed-By: Amit Langote
Backpatch-through: 11 where partitioned index was introduced
Discussion: https://postgr.es/m/1553869152.858391073.5f8m3n0x@frv53.fwdcdn.com

Author: Etsuro Fujita

doc/src/sgml/ref/alter_table.sgml

@@ -842,6 +842,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       one will be created in the attached table; or, if an equivalent
       index already exists, will be attached to the target table's index,
       as if <command>ALTER INDEX ATTACH PARTITION</command> had been executed.
+      Note that if the existing table is a foreign table, it is currently not
+      allowed to attach the table as a partition of the target table if there
+      are indexes on the target table.  (See also
+      <xref linkend="sql-createforeigntable"/>.)
      </para>
 
      <para>

doc/src/sgml/ref/create_foreign_table.sgml

@@ -169,6 +169,10 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
       the given parent table with specified partition bound values.
       See the similar form of
       <xref linkend="sql-createtable"/> for more details.
+      Note that it is currently not allowed to create the foreign table as a
+      partition of the parent table if there are indexes on the parent table.
+      (See also
+      <link linkend="sql-altertable"><command>ALTER TABLE ATTACH PARTITION</command></link>.)
      </para>
     </listitem>
    </varlistentry>