pgsql: Add comment about checkpoint panic behavior during shutdown, per

Log Message:
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Add comment about checkpoint panic behavior during shutdown, per
suggestion from Qingqing Zhou.

Modified Files:
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pgsql/src/backend/access/transam:
xlog.c (r1.187 -> r1.188)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.187&r2=1.188)