bench, doc: benchmarking updates and fixups

Summary:
Backport of core#22292.

Depends on D12900.

Test Plan:

ninja bench-bitcoin
./src/bench/bitcoin-bench -?

Read the doc.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12901