acme/autocert: extract Manager example from comments

Ensures that it compiles and makes it more discoverable in godoc.

Change-Id: I50a3deb23110017d0eb16b1da0ac729a96ff2602
Reviewed-on: https://go-review.googlesource.com/40952
Run-TryBot: Alex Vaghin <ddos@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

acme/autocert/autocert.go

@@ -77,18 +77,6 @@ func defaultHostPolicy(context.Context, string) error {
 // It obtains and refreshes certificates automatically,
 // as well as providing them to a TLS server via tls.Config.
 //
-// A simple usage example:
-//
-//	m := autocert.Manager{
-//		Prompt: autocert.AcceptTOS,
-//		HostPolicy: autocert.HostWhitelist("example.org"),
-//	}
-//	s := &http.Server{
-//		Addr: ":https",
-//		TLSConfig: &tls.Config{GetCertificate: m.GetCertificate},
-//	}
-//	s.ListenAndServeTLS("", "")
-//
 // To preserve issued certificates and improve overall performance,
 // use a cache implementation of Cache. For instance, DirCache.
 type Manager struct {

acme/autocert/listener_test.go → acme/autocert/example_test.go

@@ -5,6 +5,7 @@
 package autocert_test
 
 import (
+	"crypto/tls"
 	"fmt"
 	"log"
 	"net/http"
@@ -19,3 +20,15 @@ func ExampleNewListener() {
 	})
 	log.Fatal(http.Serve(autocert.NewListener("example.com"), mux))
 }
+
+func ExampleManager() {
+	m := autocert.Manager{
+		Prompt:     autocert.AcceptTOS,
+		HostPolicy: autocert.HostWhitelist("example.org"),
+	}
+	s := &http.Server{
+		Addr:      ":https",
+		TLSConfig: &tls.Config{GetCertificate: m.GetCertificate},
+	}
+	s.ListenAndServeTLS("", "")
+}