sort: update main example to use Slice along with Sort

This is to let sort.Slice get more prominence since
it's the most common use case.

Fixes #21989

Change-Id: I0b180cc20256f5f09065b722e191c508c872f4f7
Reviewed-on: https://go-review.googlesource.com/65710Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/sort/example_interface_test.go

@@ -35,10 +35,24 @@ func Example() {
 	}
 
 	fmt.Println(people)
+	// There are two ways to sort a slice. First, one can define
+	// a set of methods for the slice type, as with ByAge, and
+	// call sort.Sort. In this first example we use that technique.
 	sort.Sort(ByAge(people))
 	fmt.Println(people)
 
+	// The other way is to use sort.Slice with a custom Less
+	// function, which can be provided as a closure. In this
+	// case no methods are needed. (And if they exist, they
+	// are ignored.) Here we re-sort in reverse order: compare
+	// the closure with ByAge.Less.
+	sort.Slice(people, func(i, j int) bool {
+		return people[i].Age > people[j].Age
+	})
+	fmt.Println(people)
+
 	// Output:
 	// [Bob: 31 John: 42 Michael: 17 Jenny: 26]
 	// [Michael: 17 Jenny: 26 Bob: 31 John: 42]
+	// [John: 42 Bob: 31 Jenny: 26 Michael: 17]
 }