net/textproto: add Conn to doc comment

This revolutionary transfiguration undergirding the code base
Fixes #2945.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5676072

src/pkg/net/textproto/textproto.go

@@ -20,6 +20,9 @@
 //
 // Writer, to write dot-encoded text blocks.
 //
+// Conn, a convenient packaging of Reader, Writer, and Pipeline for use
+// with a single network connection."
+//
 package textproto
 
 import (