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

apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,tv42/old-go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go