net/http: document that after Hijack, Request.Body is invalid

We can make it panic with a more explicit and readable error message
during Go 1.10, but document it for now. This has always been the
case; it's not a new rule.

Updates #20933

Change-Id: I53c1fefb47a8f4aae0bb32fa742afa3a2ed20e8a
Reviewed-on: https://go-review.googlesource.com/50634Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/http/server.go

@@ -178,6 +178,9 @@ type Hijacker interface {
 	//
 	// The returned bufio.Reader may contain unprocessed buffered
 	// data from the client.
+	//
+	// After a call to Hijack, the original Request.Body should
+	// not be used.
 	Hijack() (net.Conn, *bufio.ReadWriter, error)
 }