doc: add PerformanceObserver `buffered` document

The option buffered is not about queueing the PerformanceEntrys with
an event loop task or not. The option buffered in the spec is about
filling the observer with the global PerformanceEntry buffer. The
current (and the spec) behavior is different with Node.js
version <= v16.0.0.

PR-URL: https://github.com/nodejs/node/pull/39514
Refs: https://w3c.github.io/performance-timeline/#observe-method
Refs: https://nodejs.org/dist/latest-v14.x/docs/api/perf_hooks.html#perf_hooks_performanceobserver_observe_options
Refs: https://github.com/nodejs/node/pull/39297


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>