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.

Fixes: nodejs#19071

Documentation for napi_create_async_work was missing part of the word "completed" in the parameter documentation

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.