doc: update napi_async_init documentation

Fixes: https://github.com/nodejs/node/issues/33153



Change documentation to make async_resource required
as opposed to optional in napi-async_init.

Changes over time mean this parameter is required for
proper operation of async hooks (which are still experimental).
This changes the documentation to document what
callers should do. We are doing this only in the doc
in order to avoid a breaking change in N-API. We could
create a new version of the method for which the
parametrer is enforced as mandatory but we should only
do that once async hooks is no longer experimental. In
that case we could deprecate (but not remove this version
of the method).

Signed-off-by: Michael Dawson <michael_dawson@ca.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/33181


Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>