doc: clarify about the Node.js-only extensions in perf_hooks

- Add clarifications for Node.js-only extensions
- Explain the Web Performance APIs implemented in Node.js and
  clarify that perf_hooks also include other non-Web APIs.
- Prefix exposed interfaces with `perf_hooks.` to distinguish
  them from internal classes.

PR-URL: https://github.com/nodejs/node/pull/33199
Refs: https://github.com/nodejs/node/issues/28635


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>