pgsql: Document that 'sslmode' is ignored for Unix domain socket

Log Message:
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Document that 'sslmode' is ignored for Unix domain socket communication;
backpatch to 8.3.X.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/doc/src/sgml:
libpq.sgml (r1.255.2.1 -> r1.255.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.255.2.1&r2=1.255.2.2)