http: document that Response.Body is non-nil

Fixes #2208

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5011051

src/pkg/http/client.go

@@ -76,7 +76,12 @@ type readClose struct {
 // Do sends an HTTP request and returns an HTTP response, following
 // policy (e.g. redirects, cookies, auth) as configured on the client.
 //
-// Callers should close resp.Body when done reading from it.
+// A non-nil response always contains a non-nil resp.Body.
+//
+// Callers should close resp.Body when done reading from it. If
+// resp.Body is not closed, the Client's underlying RoundTripper
+// (typically Transport) may not be able to re-use a persistent TCP
+// connection to the server for a subsequent "keep-alive" request.
 //
 // Generally Get, Post, or PostForm will be used instead of Do.
 func (c *Client) Do(req *Request) (resp *Response, err os.Error) {

src/pkg/http/response.go

@@ -41,6 +41,10 @@ type Response struct {
 	Header Header
 
 	// Body represents the response body.
+	//
+	// The http Client and Transport guarantee that Body is always
+	// non-nil, even on responses without a body or responses with
+	// a zero-lengthed body.
 	Body io.ReadCloser
 
 	// ContentLength records the length of the associated content.  The