http/httpproxy: document all loopbacks are special cased not just localhost

Fixes golang/go#33695

Change-Id: I2f7598da7c6fda161ba357a75bba72de2fee8226
GitHub-Last-Rev: 443dbb1
GitHub-Pull-Request: #116
Reviewed-on: https://go-review.googlesource.com/c/net/+/356510
Reviewed-by: Damien Neil <[email protected]>
Trust: Damien Neil <[email protected]>
Trust: Cherry Mui <[email protected]>
Run-TryBot: Damien Neil <[email protected]>

Author: AlexanderYastrebov

http/httpproxy/proxy.go

@@ -113,8 +113,8 @@ func getEnvAny(names ...string) string {
 // environment, or a proxy should not be used for the given request, as
 // defined by NO_PROXY.
 //
-// As a special case, if req.URL.Host is "localhost" (with or without a
-// port number), then a nil URL and nil error will be returned.
+// As a special case, if req.URL.Host is "localhost" or a loopback address
+// (with or without a port number), then a nil URL and nil error will be returned.
 func (cfg *Config) ProxyFunc() func(reqURL *url.URL) (*url.URL, error) {
 	// Preprocess the Config settings for more efficient evaluation.
 	cfg1 := &config{