chore: Add callout for Android storage permissions

Author: vincetran

src/pages/[platform]/build-a-backend/storage/download-files/index.mdx

@@ -336,6 +336,14 @@ const { body, eTag } = await downloadData({
 
 Use the `downloadFile` API to download the file locally on the client.
 
+<Callout>
+**Note:** When downloading a file that will overwrite prexisting file, ensure that your app has the proper write permission to overwrite it. In particular, if your app uses scoped storage, your app must must request that the user grant your app write access to that particular file [as described here](https://developer.android.com/training/data-storage/shared/media#update-other-apps-files) if a different app contributed that file to the media store.
+
+To learn more, refer to Android's developer documentation about [Scoped Storage](https://developer.android.com/training/data-storage#scoped-storage).
+
+Amplify will throw a `StorageException` if it is unable to modify a preexisting file.
+</Callout>
+
 <BlockSwitcher>
 <Block name="Java">