encoding/binary: document that PutVarint, PutUvarint may panic
encoding/binary: document that PutVarint, PutUvarint may panic

Fixes issue 2628

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5571058
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go