testing: clarify how B.Loop avoids optimizing away all the useful work
As discussed in #73137, we want to clarify the description of how
B.Loop avoids surprising optimizations, while also hinting that
the exact approach might change in the future.
Updates #73137
Change-Id: I8536540cd5d79804a47fba8cd6eec3821864309d
Reviewed-on: https://go-review.googlesource.com/c/go/+/662356
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>