doc: fix up N-API doc

• Fixed the signature for napi_status napi_get_uv_event_loop from:

NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                               uv_loop_t** loop);

to:

NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                               uv_loop_s** loop);

• In section Simple Asynchronous Operations changed type napi_work to napi_async_work

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines