pgsql: Add documentation mentioning that there are separate regression

Log Message:
-----------
Add documentation mentioning that there are separate regression tests
for the PL languages and for contrib.

Modified Files:
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pgsql/doc/src/sgml:
regress.sgml (r1.47 -> r1.48)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/regress.sgml.diff?r1=1.47&r2=1.48)