docs: PG 12 release notes, mention that REINDEX could now fail

This is because of the new tid in the index entry.

Reported-by: Peter Geoghegan

doc/src/sgml/release-12.sgml

@@ -252,6 +252,12 @@ Previously, <command>\pset format a</command> chose <literal>aligned</literal>;
 </para>
 </listitem>
 
+<listitem>
+<para>
+The maximum btree index length is now reduced by eight bytes;  a <xref linkend="sql-reindex"/> operation could potentially fail.
+</para>
+</listitem>
+
 <listitem>
 <!--
 Author: Tom Lane <tgl@sss.pgh.pa.us>