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: https://github.com/nodejs/node/pull/19073
Fixes: https://github.com/nodejs/node/issues/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>