pgsql: Mention BEGIN { strict->import(); } as a substitute for 'use

Log Message:
-----------
Mention BEGIN { strict->import(); } as a substitute for 'use strict'
in plperl functions.

Modified Files:
--------------
pgsql/doc/src/sgml:
plperl.sgml (r2.44 -> r2.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml.diff?r1=2.44&r2=2.45)