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

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