doc: fix n-api asynchronous threading docs

Documentation for N-API Custom Asynchronous Operations incorrectly
stated that async execution happens on the main event loop.
Added details to napi_create_async_work about which threads are
used to invoke the execute and complete callbacks.

Changed 'async' to 'asynchronous' in the documentation for Custom
Asynchronous Operations. Changed "executes in parallel" to "can
execute in parallel" for the documentation of napi_create_async_work
execute parameter.

PR-URL: #19073
Fixes: #19071
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/n-api.md

@@ -3268,9 +3268,11 @@ napi_status napi_create_async_work(napi_env env,
 - `[in] async_resource_name`: Identifier for the kind of resource that is
 being provided for diagnostic information exposed by the `async_hooks` API.
 - `[in] execute`: The native function which should be called to excute
-the logic asynchronously.
+the logic asynchronously. The given function is called from a worker pool
+thread and can execute in parallel with the main event loop thread.
 - `[in] complete`: The native function which will be called when the
-asynchronous logic is comple or is cancelled.
+asynchronous logic is completed or is cancelled. The given function is called
+from the main event loop thread.
 - `[in] data`: User-provided data context. This will be passed back into the
 execute and complete functions.
 - `[out] result`: `napi_async_work*` which is the handle to the newly created
@@ -3346,9 +3348,9 @@ callback invocation, even if it has been successfully cancelled.
 
 ## Custom Asynchronous Operations
 The simple asynchronous work APIs above may not be appropriate for every
-scenario, because with those the async execution still happens on the main
-event loop. When using any other async mechanism, the following APIs are
-necessary to ensure an async operation is properly tracked by the runtime.
+scenario. When using any other asynchronous mechanism, the following APIs
+are necessary to ensure an asynchronous operation is properly tracked by
+the runtime.
 
 ### napi_async_init
 <!-- YAML