pgsql: More clearly document what the different sslmode options mean,

Log Message:
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More clearly document what the different sslmode options mean, both the new
and the old ones.

Consistently talk about certificate verification, and not validation.

Modified Files:
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pgsql/doc/src/sgml:
libpq.sgml (r1.286 -> r1.287)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.286&r2=1.287)