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

glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,rdp/rogerpack2005-golang,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc