Review wording on tablespaces w.r.t. partitioned tables

Remove a redundant comment, and document pg_class.reltablespace properly
in catalogs.sgml.

After commits a36c84c3e4, 87259588d0 and others.

Backpatch to 12.

Discussion: https://postgr.es/m/202403191013.w2kr7wqlamqz@alvherre.pgsql

doc/src/sgml/catalogs.sgml

@@ -2010,9 +2010,12 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        (references <link linkend="catalog-pg-tablespace"><structname>pg_tablespace</structname></link>.<structfield>oid</structfield>)
       </para>
       <para>
-       The tablespace in which this relation is stored.  If zero,
-       the database's default tablespace is implied.  (Not meaningful
-       if the relation has no on-disk file.)
+       The tablespace in which this relation is stored.
+       If zero, the database's default tablespace is implied.
+       Not meaningful if the relation has no on-disk file,
+       except for partitioned tables, where this is the tablespace
+       in which partitions will be created when one is not
+       specified in the creation command.
       </para></entry>
      </row>
 

src/backend/commands/tablecmds.c

@@ -785,10 +785,6 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	}
 	else if (stmt->partbound)
 	{
-		/*
-		 * For partitions, when no other tablespace is specified, we default
-		 * the tablespace to the parent partitioned table's.
-		 */
 		Assert(list_length(inheritOids) == 1);
 		tablespaceId = get_rel_tablespace(linitial_oid(inheritOids));
 	}