net/http: clarify some RoundTripper behaviors

Fixes #12796
Updates #13444

Change-Id: I56840c0baf9b32a683086a80f5db1c5ea0a7aedf
Reviewed-on: https://go-review.googlesource.com/17680Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/client.go

@@ -83,19 +83,26 @@ var DefaultClient = &Client{}
 // goroutines.
 type RoundTripper interface {
 	// RoundTrip executes a single HTTP transaction, returning
-	// the Response for the request req.  RoundTrip should not
-	// attempt to interpret the response.  In particular,
-	// RoundTrip must return err == nil if it obtained a response,
-	// regardless of the response's HTTP status code.  A non-nil
-	// err should be reserved for failure to obtain a response.
-	// Similarly, RoundTrip should not attempt to handle
-	// higher-level protocol details such as redirects,
+	// a Response for the provided Request.
+	//
+	// RoundTrip should not attempt to interpret the response. In
+	// particular, RoundTrip must return err == nil if it obtained
+	// a response, regardless of the response's HTTP status code.
+	// A non-nil err should be reserved for failure to obtain a
+	// response. Similarly, RoundTrip should not attempt to
+	// handle higher-level protocol details such as redirects,
 	// authentication, or cookies.
 	//
 	// RoundTrip should not modify the request, except for
-	// consuming and closing the Body, including on errors. The
-	// request's URL and Header fields are guaranteed to be
-	// initialized.
+	// consuming and closing the Request's Body.
+	//
+	// RoundTrip must always close the body, including on errors,
+	// but depending on the implementation may do so in a separate
+	// goroutine even after RoundTrip returns. This means that
+	// callers wanting to reuse the body for subsequent requests
+	// must arrange to wait for the Close call before doing so.
+	//
+	// The Request's URL and Header fields must be initialized.
 	RoundTrip(*Request) (*Response, error)
 }