math/rand: clarify documentation for Seed example

Fixes #25325

Change-Id: I101641be99a820722edb7272918e04e8d2e1646c
Reviewed-on: https://go-review.googlesource.com/112775Reviewed-by: Rob Pike <r@golang.org>

src/math/rand/example_test.go

@@ -16,7 +16,10 @@ import (
 // the output of the random number generator when given a fixed seed.
 
 func Example() {
-	rand.Seed(42) // Try changing this number!
+	// Seeding with the same value results in the same random sequence each run.
+	// For different numbers, seed with a different value, such as
+	// time.Now().UnixNano(), which yields a constantly-changing number.
+	rand.Seed(42)
 	answers := []string{
 		"It is certain",
 		"It is decidedly so",