pgsql: Mention that some psql environment variables come from libpq and

Log Message:
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Mention that some psql environment variables come from libpq and note
more variables can be found in the libpq manual section.

Mention .pgpass in the psql manual page section dealing with connection
parameters and point to the libpq section for more details.

Backpatch to 8.0.X.

Modified Files:
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pgsql/doc/src/sgml/ref:
psql-ref.sgml (r1.132 -> r1.133)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.132&r2=1.133)