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

Second try. The previous version (CL 115039 in git rev 39888635) wasn't
accurate.

Fixes #22347

Change-Id: I473165f308c730f50b14ba787cb215f7cb9ea364
Reviewed-on: https://go-review.googlesource.com/119235Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/request.go

@@ -321,8 +321,7 @@ type Request struct {
 //
 // For incoming server requests, the context is canceled when the
 // client's connection closes, the request is canceled (with HTTP/2),
-// the ServeHTTP method returns, or if the Hijack method is
-// called on the ResponseWriter.
+// or when the ServeHTTP method returns.
 func (r *Request) Context() context.Context {
 	if r.ctx != nil {
 		return r.ctx

src/net/http/server.go

@@ -190,7 +190,9 @@ type Hijacker interface {
 	// data from the client.
 	//
 	// After a call to Hijack, the original Request.Body must not
-	// be used, and the Request.Context will be canceled.
+	// be used. The original Request's Context remains valid and
+	// is not canceled until the Request's ServeHTTP method
+	// returns.
 	Hijack() (net.Conn, *bufio.ReadWriter, error)
 }