doc: document napi_finalize() signature

Refs: https://github.com/nodejs/node/issues/14138
PR-URL: https://github.com/nodejs/node/pull/14230
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

doc/api/n-api.md

@@ -181,6 +181,12 @@ must provide a function satisfying the following signature which would get
 called upon the object's collection. Currently, `napi_finalize` can be used for
 finding out when objects that have external data are collected.
 
+```C
+typedef void (*napi_finalize)(napi_env env,
+                              void* finalize_data,
+                              void* finalize_hint);
+```
+
 
 #### napi_async_execute_callback
 Function pointer used with functions that support asynchronous