doc: added some warnings for buffer and array buffer factory method. (#929)

* Fixes issue 258.

Author: NickNaso

doc/array_buffer.md

@@ -29,6 +29,12 @@ The `Napi::ArrayBuffer` instance does not assume ownership for the data and
 expects it to be valid for the lifetime of the instance. Since the
 `Napi::ArrayBuffer` is subject to garbage collection this overload is only
 suitable for data which is static and never needs to be freed.
+This factory method will not provide the caller with an opportunity to free the
+data when the `Napi::ArrayBuffer` gets garbage-collected. If you need to free
+the data retained by the `Napi::ArrayBuffer` object please use other
+variants of the `Napi::ArrayBuffer::New` factory method that accept
+`Napi::Finalizer`, which is a function that will be invoked when the
+`Napi::ArrayBuffer` object has been destroyed.
 
 ```cpp
 static Napi::ArrayBuffer Napi::ArrayBuffer::New(napi_env env, void* externalData, size_t byteLength);

doc/buffer.md

@@ -28,6 +28,12 @@ The `Napi::Buffer` object does not assume ownership for the data and expects it
 valid for the lifetime of the object. Since the `Napi::Buffer` is subject to garbage
 collection this overload is only suitable for data which is static and never
 needs to be freed.
+This factory method will not provide the caller with an opportunity to free the
+data when the `Napi::Buffer` gets garbage-collected. If you need to free the
+data retained by the `Napi::Buffer` object please use other variants of the
+`Napi::Buffer::New` factory method that accept `Napi::Finalizer`, which is a
+function that will be invoked when the `Napi::Buffer` object has been
+destroyed.
 
 ```cpp
 static Napi::Buffer<T> Napi::Buffer::New(napi_env env, T* data, size_t length);