testing: mention that TB also covers the new type F

Fixes #48146

Change-Id: I7c667a7915db81558514bc9fada6898c565eb0fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/365894
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Katie Hockman <[email protected]>
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Katie Hockman <[email protected]>
Trust: Ian Lance Taylor <[email protected]>

Author: uppalabharath

src/testing/testing.go

@@ -740,7 +740,7 @@ func fmtDuration(d time.Duration) string {
 	return fmt.Sprintf("%.2fs", d.Seconds())
 }
 
-// TB is the interface common to T and B.
+// TB is the interface common to T, B, and F.
 type TB interface {
 	Cleanup(func())
 	Error(args ...interface{})