doc: PG 12 relnotes, list added snowball/FTS languages

Reported-by: Adrien Nayrat

Discussion: https://postgr.es/m/9d6a7515-bcd8-05be-d2a5-e81dc11023cd@anayrat.info

doc/src/sgml/release-12.sgml

@@ -2077,8 +2077,8 @@ Author: Tom Lane <tgl@sss.pgh.pa.us>
       </para>
 
       <para>
-       This adds additional stemming languages to <link
-       linkend="textsearch">full text search</link>.
+       This adds stemming for Arabic, Indonesian, Irish, Lithuanian, Nepali,
+       and Tami to <link linkend="textsearch">full text search</link>.
       </para>
      </listitem>