docs: adding a reader is not the same as adopting it

Records the macOS backend defect and, more usefully, how to find the next one.
`platform::macos::read_cpu_stats` was added in 5.2.0 and wired into the ontology
alone; `MonitoringBackend` kept fabricating for two releases, so one Mac had two
CPU paths that disagreed and nothing compared them.

Also records that the sweep which found it -- grepping for `assume`,
`approximation`, `hardcoded`, `placeholder`, `requires admin` next to returned
values -- has not been run over the whole crate. Four modules still carry such
comments and have not been examined.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>