bufio: discourage use of ReadLine
bufio: discourage use of ReadLine

Too many people use it without reading what it does.
Those people want ReadBytes or ReadString.

Fixes issue 3906.

R=golang-dev, iant, r
CC=golang-dev
http://codereview.appspot.com/6442087
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go