net/http: document that body is closed after (*Request).Write.

Fixes #6911.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/38690043

src/pkg/net/http/response.go

@@ -187,6 +187,7 @@ func (r *Response) ProtoAtLeast(major, minor int) bool {
 //  ContentLength
 //  Header, values for non-canonical keys will have unpredictable behavior
 //
+// Body is closed after it is sent.
 func (r *Response) Write(w io.Writer) error {
 
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