further explain the correspondence between the ECMAScript Agent and t…

…he Node.js environment

doc/api/n-api.md

@@ -260,6 +260,15 @@ of an "Agent" as a self-contained environment in which JavaScript code runs.
 Multiple such Agents may be started and terminated either concurrently or in
 sequence by the process.
 
+A Node.js environment corresponds to an ECMAScript Agent. In the main process,
+an environment is created at startup, and additional environments can be created
+on separate threads to serve as [worker threads][]. When Node.js is embedded in
+another application, the main thread of the application may also construct and
+destroy a Node.js environment multiple times during the life cycle of the
+application process such that each Node.js environment created by the
+application may, in turn, during its life cycle create and destroy additional
+environments as worker threads.
+
 From the perspective of a native addon this means that the bindings it provides
 may be called multiple times, from multiple contexts, and even concurrently from
 multiple threads.
@@ -4994,3 +5003,4 @@ This API may only be called from the main thread.
 [`uv_unref`]: http://docs.libuv.org/en/v1.x/handle.html#c.uv_unref
 [async_hooks `type`]: async_hooks.html#async_hooks_type
 [context-aware addons]: addons.html#addons_context_aware_addons
+[worker threads]: https://nodejs.org/api/worker_threads.html