net/http/httptrace: clarify WroteRequest may be called multiple times

Updates #18305

Change-Id: I63b28d511df1a6c54e32c8bfc7e2264f94e38cd7
Reviewed-on: https://go-review.googlesource.com/34386
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/httptrace/trace.go

@@ -146,7 +146,8 @@ type ClientTrace struct {
 	Wait100Continue func()
 
 	// WroteRequest is called with the result of writing the
-	// request and any body.
+	// request and any body. It may be called multiple times
+	// in the case of retried requests.
 	WroteRequest func(WroteRequestInfo)
 }