net/http: clarify proxy selection from environment

For #40909
Fixes #54890

Change-Id: I00218bc1606eedb6194a3a7b81fd4d3f75325280
Reviewed-on: https://go-review.googlesource.com/c/go/+/428775
Reviewed-by: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Damien Neil <[email protected]>

Author: seankhliao

src/net/http/transport.go

@@ -422,8 +422,8 @@ func (t *Transport) onceSetNextProtoDefaults() {
 // ProxyFromEnvironment returns the URL of the proxy to use for a
 // given request, as indicated by the environment variables
 // HTTP_PROXY, HTTPS_PROXY and NO_PROXY (or the lowercase versions
-// thereof). HTTPS_PROXY takes precedence over HTTP_PROXY for https
-// requests.
+// thereof). Requests use the proxy from the environment variable
+// matching their scheme, unless excluded by NO_PROXY.
 //
 // The environment values may be either a complete URL or a
 // "host[:port]", in which case the "http" scheme is assumed.