pgsql: Update float dpow() comment about whick platforms had issues with

Log Message:
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Update float dpow() comment about whick platforms had issues with Nan.

Stefan Kaltenbrunner

Modified Files:
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pgsql/src/backend/utils/adt:
float.c (r1.140 -> r1.141)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/float.c.diff?r1=1.140&r2=1.141)