pgsql: Document in postgresql.conf that the max_standby_delay default is

Log Message:
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Document in postgresql.conf that the max_standby_delay default is 30
'seconds'.

Modified Files:
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pgsql/src/backend/utils/misc:
postgresql.conf.sample (r1.279 -> r1.280)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.279&r2=1.280)