crypto/rand: simplify example to exploit properties of ReadFull
No need for the complex condition.
Fixes issue 6089

R=golang-dev, mischief, adg
CC=golang-dev
https://codereview.appspot.com/12731043
bufio: make it clear that the client must call Writer.Flush

Fixes issue 5530.

R=golang-dev, iant, com.liigo
CC=golang-dev
https://codereview.appspot.com/12688044