doc: mention null special-case for `napi_typeof`

The documentation said `napi_typeof` is similar to the `typeof`
operator, but the `typeof null` detects `'object'` while
`napi_typeof(a_null_value)` detects `napi_null`.

PR-URL: https://github.com/nodejs/node/pull/34577


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>