pgsql: Document that datetime year '0' is considered in a recent

Log Message:
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Document that datetime year '0' is considered in a recent century, not
just '00'.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
datetime.c (r1.200 -> r1.201)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.200&r2=1.201)