doc: update napi_make_callback documentation

Calling napi_make_callback() with no async_context is not resulting in
using the current async context instead an empty context (id 0) is
used.

Using NULL is like using node::Makecallback without async_context which
is deprecated since Node.js 10 (DEP0099).

PR-URL: https://github.com/nodejs/node/pull/35321
Fixes: https://github.com/nodejs/node/issues/35188


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>