testing: document -test.bench flag

Fixes #4080.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/6553068

src/pkg/testing/testing.go

@@ -15,6 +15,9 @@
 // are considered benchmarks, and are executed by the "go test" command when
 // the -test.bench flag is provided. Benchmarks are run sequentially.
 // 
+// For a description of the testing flags, see
+// http://golang.org/cmd/go/#Description_of_testing_flags.
+//
 // A sample benchmark function looks like this:
 //     func BenchmarkHello(b *testing.B) {
 //         for i := 0; i < b.N; i++ {