testing: improve B.Loop docs, use B.Loop in examples

This updates the testing documentation to frame B.Loop as the
canonical way to write benchmarks. We retain documentation on b.N
benchmarks because people will definitely continue to see them (and
write them), but it's demoted to clearly second class.

This also attempts to clarify and refine the B.Loop documentation
itself.

Updates #61515
Fixes #70787

Change-Id: If5123435bfe3a5883a753119ecdf7bbc41afd499
Reviewed-on: https://go-review.googlesource.com/c/go/+/635895
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Caleb Spare <cespare@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>

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