net/http: clarify DefaultTransport docs
net/http: clarify DefaultTransport docs

Fixes issue 4281

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/6872053
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go