doc: improve PG 12 item on partitioned tables

Reported-by: Amit Langote

Discussion: https://postgr.es/m/5936b052-5d92-a2c9-75d2-0245fb2330b5@lab.ntt.co.jp

doc/src/sgml/release-12.sgml

@@ -359,11 +359,11 @@ Author: Tom Lane <tgl@sss.pgh.pa.us>
 -->
 
 <para>
-Improve performance of pruning many partitions (Amit Langote, David Rowley, Tom Lane, Álvaro Herrera)
+Improve performance of many operations on partitioned tables (Amit Langote, David Rowley, Tom Lane, Álvaro Herrera)
 </para>
 
 <para>
-Now thousands of partitions can be pruned efficiently.
+Also, thousands of partitions can now be pruned efficiently.
 </para>
 </listitem>