Improve documentation for default_tablespace on partitioned tables

Backpatch to 12, where 87259588d0 introduced the current behavior.

Per note from Justin Pryzby.

Co-authored-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20210416143135.GI3315@telsasoft.com

doc/src/sgml/config.sgml

@@ -8268,8 +8268,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         This variable specifies the default tablespace in which to create
         objects (tables and indexes) when a <command>CREATE</command> command does
-        not explicitly specify a tablespace.  It also determines the tablespace
-        that a partitioned relation will direct future partitions to.
+        not explicitly specify a tablespace.
        </para>
 
        <para>
@@ -8293,6 +8292,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         the template database it is copied from.
        </para>
 
+       <para>
+        If this parameter is set to a value other than the empty string
+        when a partitioned table is created, the partitioned table's
+        tablespace will be set to that value, which will be used as
+        the default tablespace for partitions created in the future,
+        even if <varname>default_tablespace</varname> has changed since then.
+       </para>
+
        <para>
         For more information on tablespaces,
         see <xref linkend="manage-ag-tablespaces"/>.