Fix debugging-log documentation errors.

ART has never supported log.redirect-stdio.

Don't imply that System.out and System.err go to /dev/null.

Bug: http://b/18092307
Bug: https://code.google.com/p/android/issues/detail?id=165602
Bug: https://code.google.com/p/android/issues/detail?id=62555
Change-Id: If5304a129e8941865211deb86766531925255796