pgsql: Move libpq environment reference in psql documentation to a more

Log Message:
-----------
Move libpq environment reference in psql documentation to a more
appropriate place, per Andrew Dunstan.

Modified Files:
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pgsql/doc/src/sgml/ref:
psql-ref.sgml (r1.174 -> r1.175)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.174&r2=1.175)