io: document ReaderFrom and WriterTo
io: document ReaderFrom and WriterTo

Fixes issue 3711

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/6445083

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