testing: improve the documentation around b.N

testing: improve the documentation around b.N

Fixes #67137

- Make it clear the benchmark function is called multiple times.
- Demonstrate range over int.

Change-Id: I7e993d938b0351012cdd4aed8528951e0ad406ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/582835
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>