net/http: document streaming nature of Response.Body

Fixes #22873

Change-Id: Ib2b7ee42a23b84db21cdfa693b62d5e6fbfdb54e
Reviewed-on: https://go-review.googlesource.com/80075Reviewed-by: Tom Bergan <tombergan@google.com>

src/net/http/response.go

@@ -27,6 +27,9 @@ var respExcludeHeader = map[string]bool{
 
 // Response represents the response from an HTTP request.
 //
+// The Client and Transport return Responses from servers once
+// the response headers have been received. The response body
+// is streamed on demand as the Body field is read.
 type Response struct {
 	Status     string // e.g. "200 OK"
 	StatusCode int    // e.g. 200
@@ -47,6 +50,10 @@ type Response struct {
 
 	// Body represents the response body.
 	//
+	// The response body is streamed on demand as the Body field
+	// is read. If the network connection fails or the server
+	// terminates the response, Body.Read calls return an error.
+	//
 	// The http Client and Transport guarantee that Body is always
 	// non-nil, even on responses without a body or responses with
 	// a zero-length body. It is the caller's responsibility to