net/textproto: add Conn to doc comment
net/textproto: add Conn to doc comment

This revolutionary transfiguration undergirding the code base
Fixes issue 2945.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5676072

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