os/signal: improve documentation for the Notify function

It is easy to miss the documentation information that no arguments
in the Notify function means that the Notify will catch all possible signals.
So the example was added with explicit comment above the Notify usage.

Fixes #22257

Change-Id: Ia6a16dd4a419f7c77d89020ca5db85979b5b474e
Reviewed-on: https://go-review.googlesource.com/74730
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/signal/example_test.go

@@ -21,3 +21,18 @@ func ExampleNotify() {
 	s := <-c
 	fmt.Println("Got signal:", s)
 }
+
+func ExampleNotify_allSignals() {
+	// Set up channel on which to send signal notifications.
+	// We must use a buffered channel or risk missing the signal
+	// if we're not ready to receive when the signal is sent.
+	c := make(chan os.Signal, 1)
+
+	// Passing no signals to Notify means that
+	// all signals will be sent to the channel.
+	signal.Notify(c)
+
+	// Block until any signal is received.
+	s := <-c
+	fmt.Println("Got signal:", s)
+}