gob: explain that Debug isn't useful unless it's compiled in.
gob: explain that Debug isn't useful unless it's compiled in.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4930044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go