RFR 9:8148775 : Spec for j.l.ProcessBuilder.Redirect.DISCARD need to be improved

[Roger Riggs]

Please review a minor javadoc cleanup to remove a reference to a method  
(append)
of the implementation.

Webrev:
   http://cr.openjdk.java.net/~rriggs/webrev-discard-8148775/
Issue:
  https://bugs.openjdk.java.net/browse/JDK-8148775

Thanks, Roger