update docs

docs/api/native-image.md

@@ -163,12 +163,17 @@ Returns a [Buffer][buffer] that contains the image's `JPEG` encoded data.
 
 #### `image.toBitmap()`
 
-Returns a [Buffer][buffer] that contains the image's raw pixel data.
+Returns a [Buffer][buffer] that contains a copy of the image's raw pixel data.
 
 #### `image.toDataURL()`
 
 Returns the data URL of the image.
 
+#### `image.getBitmap()`
+
+Returns a [Buffer][buffer] that contains the image's raw pixel data. The pixel 
+data is not owned by the `Buffer` object.
+
 #### `image.getNativeHandle()` _macOS_
 
 Returns a [Buffer][buffer] that stores C pointer to underlying native handle of