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: #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>

doc/api/n-api.md

@@ -3125,7 +3125,12 @@ Returns `napi_ok` if the API succeeded.
 
 This API represents behavior similar to invoking the `typeof` Operator on
 the object as defined in [Section 12.5.5][] of the ECMAScript Language
-Specification. However, it has support for detecting an External value.
+Specification. However, there are some differences:
+
+1. It has support for detecting an External value.
+2. It detects `null` as a separate type, while ECMAScript `typeof` would detect
+   `object`.
+
 If `value` has a type that is invalid, an error is returned.
 
 ### napi_instanceof