net/url: document and add example for ParseQuery("x")

Fixes #16460.

Change-Id: Ie9d5f725d2d7e8210ab6f7604a5a05fc49f707de
Reviewed-on: https://go-review.googlesource.com/31331
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/example_test.go

@@ -5,6 +5,7 @@
 package url_test
 
 import (
+	"encoding/json"
 	"fmt"
 	"log"
 	"net/http"
@@ -98,3 +99,21 @@ func ExampleURL_ResolveReference() {
 	// Output:
 	// http://example.com/search?q=dotnet
 }
+
+func ExampleParseQuery() {
+	m, err := url.ParseQuery(`x=1&y=2&y=3;z`)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(toJSON(m))
+	// Output:
+	// {"x":["1"], "y":["2", "3"], "z":[""]}
+}
+
+func toJSON(m interface{}) string {
+	js, err := json.Marshal(m)
+	if err != nil {
+		log.Fatal(err)
+	}
+	return strings.Replace(string(js), ",", ", ", -1)
+}