net/http/httputil: remove os.Stderr claim in ReverseProxy.ErrorLog docs
net/http/httputil: remove os.Stderr claim in ReverseProxy.ErrorLog docs

The motivation for doing so is to avoid making inaccurate claims.
Logging may not go to os.Stderr if anyone overrides the log package's
default output via https://godoc.org/log#SetOutput. Saying that
the standard logger is used should be sufficient to explain the
behavior, and users can infer that os.Stderr is used by default,
unless it's changed.

This change is the same as what was applied to http.Server.ErrorLog
documentation in CL 53950.

Change-Id: I32873fc548ceee573f8616b4d49b8a8b98881803
Reviewed-on: https://go-review.googlesource.com/c/go/+/176817
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,golang/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,golang/go,golang/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go