pgsql: Update some obsolete comments --- code is using t_self now, not

Log Message:
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Update some obsolete comments --- code is using t_self now, not t_ctid.

Modified Files:
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pgsql/src/backend/executor:
execMain.c (r1.252 -> r1.253)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.252&r2=1.253)