net/http: mention socks5 support in proxy

Change-Id: I55b6d5c77221569eeafea625379affd476a65772
Reviewed-on: https://go-review.googlesource.com/c/go/+/326011
Reviewed-by: Damien Neil <dneil@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

Author: seankhliao

src/net/http/transport.go

@@ -427,6 +427,7 @@ func (t *Transport) onceSetNextProtoDefaults() {
 //
 // The environment values may be either a complete URL or a
 // "host[:port]", in which case the "http" scheme is assumed.
+// The schemes "http", "https", and "socks5" are supported.
 // An error is returned if the value is a different form.
 //
 // A nil URL and nil error are returned if no proxy is defined in the