pgsql: Add documentation of the fact that dtrace probes evaluate their

Log Message:
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Add documentation of the fact that dtrace probes evaluate their parameters
even when not active. Explain how to prevent that with an ENABLED() check.

Modified Files:
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pgsql/doc/src/sgml:
monitoring.sgml (r1.65 -> r1.66)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.65&r2=1.66)