net/url: clarify documentation about (*URL).String

Fixes #23669

Change-Id: Ib7f0aab0b066f778a3097583f432f8092310fb81
Reviewed-on: https://go-review.googlesource.com/c/155598Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/url.go

@@ -755,6 +755,7 @@ func validOptionalPort(port string) bool {
 //
 // If u.Opaque is non-empty, String uses the first form;
 // otherwise it uses the second form.
+// Any non-ASCII characters in host are escaped.
 // To obtain the path, String uses u.EscapedPath().
 //
 // In the second form, the following rules apply: