doc: document common pattern for instanceof checks

Fixes: nodejs#13824

Author: mhdawson

doc/api/n-api.md

@@ -3021,6 +3021,29 @@ constructor and methods can be called from JavaScript.
     callback, [`napi_unwrap`][] obtains the C++ instance that is the target of
     the call.
 
+For wrapped objects it may be difficult to distinguish between a function
+called on a class prototype and a function called on an instance of a class.
+A common pattern used to address this problem is to save a persistent
+reference to the class constructor for later `instanceof` checks.
+
+As an example:
+
+```
+napi_value MyClass_constructor = nullptr;
+status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);
+assert(napi_ok == status);
+bool is_instance = false;
+status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
+assert(napi_ok == status);
+if (is_instance) {
+    // napi_unwrap() ...
+} else {
+    // otherwise...
+}
+```
+
+Of course you also need to make sure to free the reference once it is no longer needed.
+
 ### *napi_define_class*
 <!-- YAML
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