doc: PG 13 relnotes, update TOAST item to mention decompression

Reported-by: Andrey M. Borodin

Discussion: https://postgr.es/m/D49B37B1-E2B9-4F67-8C6C-5CFD4015E8C5@yandex-team.ru

Author: bmomjian

doc/src/sgml/release-13.sgml

@@ -675,7 +675,7 @@ Author: Tomas Vondra <tomas.vondra@postgresql.org>
 -->
 
 <para>
-Improve retrieving of only the leading bytes of TOAST values (Binguo Bao)
+Improve speed of TOAST decompression and the retrievel of only the leading bytes of TOAST values (Binguo Bao, Andrey Borodin)
 </para>
 
 <para>