doc: add PerformanceObserver buffered document

The option `buffered` is not about queueing the `PerformanceEntry`s with
loop or not. The current (and the spec) behavior is different with
Node.js version <= v16.0.0

doc/api/perf_hooks.md

@@ -589,6 +589,10 @@ changes:
     pr-url: https://github.com/nodejs/node/pull/37136
     description: Updated to conform to User Timing Level 3. The
                  buffered option has been removed.
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/39297
+    description: Updated to conform to Performance Timeline Level 2. The
+                 buffered option has been added back.
 -->
 
 * `options` {Object}
@@ -597,6 +601,10 @@ changes:
   * `entryTypes` {string[]} An array of strings identifying the types of
     {PerformanceEntry} instances the observer is interested in. If not
     provided an error will be thrown.
+  * `buffered` {boolean} If true, the observer callback is called with a
+    list global `PerformanceEntry` buffered entries. If false, only
+    `PerformanceEntry`s created after the time point are sent to the
+    observer callback. **Default:** `false`.
 
 Subscribes the {PerformanceObserver} instance to notifications of new
 {PerformanceEntry} instances identified either by `options.entryTypes`