doc: document PerformanceNodeTiming.environment field

PR-URL: nodejs#28280
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/perf_hooks.md

@@ -219,6 +219,16 @@ The high resolution millisecond timestamp at which the Node.js process
 completed bootstrapping. If bootstrapping has not yet finished, the property
 has the value of -1.
 
+### performanceNodeTiming.environment
+<!-- YAML
+added: v8.5.0
+-->
+
+* {number}
+
+The high resolution millisecond timestamp at which the Node.js environment was
+initialized.
+
 ### performanceNodeTiming.loopExit
 <!-- YAML
 added: v8.5.0