diff --git a/src/testing/testing.go b/src/testing/testing.go
index a38b40e38d..5fd153954d 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -14,12 +14,19 @@
 // Within these functions, use the Error, Fail or related methods to signal failure.
 //
 // To write a new test suite, create a file whose name ends _test.go that
-// contains the TestXxx functions as described here. Put the file in the same
-// package as the one being tested. The file will be excluded from regular
+// contains the TestXxx functions as described here.
+// The file will be excluded from regular
 // package builds but will be included when the "go test" command is run.
-// For more detail, run "go help test" and "go help testflag".
 //
-// A simple test function looks like this:
+// The test file can be in the same package as the one being tested,
+// or in a corresponding package with the suffix "_test".
+//
+// If the test file is in the same package, it may refer to unexported
+// identifiers within the package, as in this example:
+//
+//	package abs
+//
+//	import "testing"
 //
 //	func TestAbs(t *testing.T) {
 //	    got := Abs(-1)
@@ -28,6 +35,27 @@
 //	    }
 //	}
 //
+// If the file is in a separate "_test" package, the package being tested
+// must be imported explicitly and only its exported identifiers may be used.
+// This is known as "black box" testing.
+//
+//	package abs_test
+//
+//	import (
+//		"testing"
+//
+//		"path_to_pkg/abs"
+//	)
+//
+//	func TestAbs(t *testing.T) {
+//	    got := abs.Abs(-1)
+//	    if got != 1 {
+//	        t.Errorf("Abs(-1) = %d; want 1", got)
+//	    }
+//	}
+//
+// For more detail, run "go help test" and "go help testflag".
+//
 // # Benchmarks
 //
 // Functions of the form