docs: update partition item in PG 12 release notes

Reported-by: Amit Langote

Discussion: https://postgr.es/m/b7954643-41ef-a174-479d-1f8d4834f40a@lab.ntt.co.jp

Author: bmomjian

doc/src/sgml/release-12.sgml

@@ -374,7 +374,9 @@ Improve performance of many operations on partitioned tables (Amit Langote, Davi
 </para>
 
 <para>
-Tables with thousands of child partitions can now be processed efficiently.
+Tables with thousands of child partitions can now be processed
+efficiently by operations that only need to touch a small number
+of partitions.
 </para>
 </listitem>