pgsql: Remove some verbiage describing how min() and max() are slow when

Log Message:
-----------
Remove some verbiage describing how min() and max() are slow when applied
to the entire table: as of current sources, they are no longer slow
provided there is an index on the column.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.249 -> r1.250)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.249&r2=1.250)