net/http: better stand-alone FileServer doc example

Motivated by a deletion in the wiki, which had a better
example.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8288045

src/pkg/net/http/example_test.go

@@ -51,11 +51,20 @@ func ExampleGet() {
 }
 
 func ExampleFileServer() {
-	// we use StripPrefix so that /tmpfiles/somefile will access /tmp/somefile
+	// Simple static webserver:
+	log.Fatal(http.ListenAndServe(":8080", http.FileServer(http.Dir("/usr/share/doc"))))
+}
+
+func ExampleFileServer_stripPrefix() {
+	// To serve a directory on disk (/tmp) under an alternate URL
+	// path (/tmpfiles/), use StripPrefix to modify the request
+	// URL's path before the FileServer sees it:
 	http.Handle("/tmpfiles/", http.StripPrefix("/tmpfiles/", http.FileServer(http.Dir("/tmp"))))
 }
 
 func ExampleStripPrefix() {
-	// we use StripPrefix so that /tmpfiles/somefile will access /tmp/somefile
+	// To serve a directory on disk (/tmp) under an alternate URL
+	// path (/tmpfiles/), use StripPrefix to modify the request
+	// URL's path before the FileServer sees it:
 	http.Handle("/tmpfiles/", http.StripPrefix("/tmpfiles/", http.FileServer(http.Dir("/tmp"))))
 }