pgsql: Improve wording of new documentation section on encryption, and

Log Message:
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Improve wording of new documentation section on encryption, and move it
a few sections up.

Modified Files:
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pgsql/doc/src/sgml:
runtime.sgml (r1.316 -> r1.317)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.316&r2=1.317)