http: document that ServerConn and ClientConn are low-level
http: document that ServerConn and ClientConn are low-level

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4635081
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go