pgsql: Add summarization comment about visibility functions.

Log Message:
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Add summarization comment about visibility functions.

Add URL about the Halloween problem.

Modified Files:
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pgsql/src/backend/utils/time:
tqual.c (r1.111 -> r1.112)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/time/tqual.c?r1=1.111&r2=1.112)