docs: PG 12 relnotes, update self-join item to mention ctid

Reported-by: Robert Haas

Discussion: https://postgr.es/m/CA+TgmoY==TZwk-4cM3Usebq1f=j7Hpm1brFbk1v48h45wsWzzg@mail.gmail.com

doc/src/sgml/release-12.sgml

@@ -835,7 +835,7 @@ Author: Tom Lane <tgl@sss.pgh.pa.us>
 -->
 
        <para>
-        Improve optimization of self-joins (Tom Lane)
+        Improve optimization of self-joins involving columns of type tid (Tom Lane)
        </para>
       </listitem>