add to documentation - upgrade from tsearch2

Hello

I migrated database cluster from 8.1 to 8.3, and I had problem with
tsvector domain. I imported tsearch2 module into template1 and then I
started import of complete dump - but public.tsvectors wasn't be known
- and I got message

ERROR: type "tsvector" is only a shell
LINE 5: fulltext_index tsvector

After some searching I found so databases are created from template0,
not template1, so my template1 modification was nonsense.

DROP DATABASE brigady;
CREATE DATABASE brigady WITH TEMPLATE = template0 ENCODING = 'UTF8';

so It needs import tsearch2.sql after every database creation

I am missing this information in documentation.

regards
Pavel Stehule