pg_upgrade: document need for text search files to be copied

Report by CJ Estel

Backpatch through 9.4

doc/src/sgml/ref/pgupgrade.sgml

@@ -274,6 +274,8 @@ make prefix=/usr/local/pgsql.new install
      whether they are from <filename>contrib</filename>
      or some other source. Do not install the schema definitions, e.g.
      <filename>pgcrypto.sql</>, because these will be upgraded from the old cluster.
+     Also, any custom full text search files (dictionary, synonym,
+     thesaurus, stop words) must also be copied to the new cluster.
     </para>
    </step>