doc: improve wording of PG 12 releaase note partition item

Reported-by: Amit Langote

Discussion: https://postgr.es/m/d5267ae5-bd4a-3e96-c21b-56bfa9fec7e8@lab.ntt.co.jp

Author: bmomjian

doc/src/sgml/release-12.sgml

@@ -374,7 +374,7 @@ Improve performance of many operations on partitioned tables (Amit Langote, Davi
 </para>
 
 <para>
-Thousands of partitions can now be pruned efficiently.
+Tables with thousands of child partitions can now be processed efficiently.
 </para>
 </listitem>