net/http: document how Hijack and Request.Context interact

Fixes #22347

Change-Id: If86aa5d54cfd7a7c32d630fb2bf4f47e057dbfb2
Reviewed-on: https://go-review.googlesource.com/115039Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/request.go

@@ -316,7 +316,8 @@ type Request struct {
 //
 // For incoming server requests, the context is canceled when the
 // client's connection closes, the request is canceled (with HTTP/2),
-// or when the ServeHTTP method returns.
+// the ServeHTTP method returns, or if the Hijack method is
+// called on the ResponseWriter.
 func (r *Request) Context() context.Context {
 	if r.ctx != nil {
 		return r.ctx

src/net/http/server.go

@@ -187,8 +187,8 @@ type Hijacker interface {
 	// The returned bufio.Reader may contain unprocessed buffered
 	// data from the client.
 	//
-	// After a call to Hijack, the original Request.Body must
-	// not be used.
+	// After a call to Hijack, the original Request.Body must not
+	// be used, and the Request.Context will be canceled.
 	Hijack() (net.Conn, *bufio.ReadWriter, error)
 }