net/http/httputil: document ProxyRequest.SetURL limitations

Fixes #50337

Change-Id: I898ff6352f46f0f9b540b053049c5116e2165827
Reviewed-on: https://go-review.googlesource.com/c/go/+/658536
Reviewed-by: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: seankhliao

src/net/http/httputil/reverseproxy.go

@@ -42,6 +42,8 @@ type ProxyRequest struct {
 // SetURL routes the outbound request to the scheme, host, and base path
 // provided in target. If the target's path is "/base" and the incoming
 // request was for "/dir", the target request will be for "/base/dir".
+// To route requests without joining the incoming path,
+// set r.Out.URL directly.
 //
 // SetURL rewrites the outbound Host header to match the target's host.
 // To preserve the inbound request's Host header (the default behavior