pgsql: Add note that TRUNCATE uses an access exclusive lock.

Log Message:
-----------
Add note that TRUNCATE uses an access exclusive lock. This apparently
surprised/confused some users.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
truncate.sgml (r1.30 -> r1.31)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/truncate.sgml?r1=1.30&r2=1.31)