net/http: clarify when it's allowed to reuse a Request

Fixes #21780

Change-Id: Ic6fb6a536fff800a05be2d25309f72092604a785
Reviewed-on: https://go-review.googlesource.com/122817
Reviewed-by: Andrew Bonventre <[email protected]>

Author: bradfitz

src/net/http/client.go

@@ -129,8 +129,8 @@ type RoundTripper interface {
 	// RoundTrip should not modify the request, except for
 	// consuming and closing the Request's Body. RoundTrip may
 	// read fields of the request in a separate goroutine. Callers
-	// should not mutate the request until the Response's Body has
-	// been closed.
+	// should not mutate or reuse the request until the Response's
+	// Body has been closed.
 	//
 	// RoundTrip must always close the body, including on errors,
 	// but depending on the implementation may do so in a separate