pgsql: Update comment to mention "Name classification hierarchy" as

Log Message:
-----------
Update comment to mention "Name classification hierarchy" as place to
check for reserved words.

Modified Files:
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pgsql/src/backend/parser:
gram.y (r2.489 -> r2.490)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.489&r2=2.490)