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

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