bufio: make it clear that the client must call Writer.Flush
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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang