net/http: document Request.Body more

Fixes #6221

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13289043

src/pkg/net/http/request.go

@@ -105,7 +105,16 @@ type Request struct {
 	// following a hyphen uppercase and the rest lowercase.
 	Header Header
 
-	// The message body.
+	// Body is the request's body.
+	//
+	// For client requests, a nil body means the request has no
+	// body, such as a GET request. The HTTP Client's Transport
+	// is responsible for calling the Close method.
+	//
+	// For server requests, the Request Body is always non-nil
+	// but will return EOF immediately when no body is present.
+	// The Server will close the request body. The ServeHTTP
+	// Handler does not need to.
 	Body io.ReadCloser
 
 	// ContentLength records the length of the associated content.