net/http: extra documentation for Redirect and RedirectHandler

Errors with http.Redirect and http.StatusOk seem
to occur from time to time on the irc channel.
This change adds documentation suggesting
to use one of the 3xx codes and not StatusOk
with Redirect.

Change-Id: I6b900a8eb868265fbbb846ee6a53e426d90a727d
Reviewed-on: https://go-review.googlesource.com/15980
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/server.go

@@ -1472,6 +1472,9 @@ func StripPrefix(prefix string, h Handler) Handler {
 
 // Redirect replies to the request with a redirect to url,
 // which may be a path relative to the request path.
+//
+// The provided code should be in the 3xx range and is usually
+// StatusMovedPermanently, StatusFound or StatusSeeOther.
 func Redirect(w ResponseWriter, r *Request, urlStr string, code int) {
 	if u, err := url.Parse(urlStr); err == nil {
 		// If url was relative, make absolute by
@@ -1556,6 +1559,9 @@ func (rh *redirectHandler) ServeHTTP(w ResponseWriter, r *Request) {
 // RedirectHandler returns a request handler that redirects
 // each request it receives to the given url using the given
 // status code.
+//
+// The provided code should be in the 3xx range and is usually
+// StatusMovedPermanently, StatusFound or StatusSeeOther.
 func RedirectHandler(url string, code int) Handler {
 	return &redirectHandler{url, code}
 }