docs: adminThumbnail feature

docs/upload/overview.mdx

@@ -42,7 +42,7 @@ Every Payload Collection can opt-in to supporting Uploads by specifying the `upl
 | **`staticURL`** *      | The base URL path to use to access you uploads. Example: `/media` |
 | **`staticDir`** *      | The folder directory to use to store media in. Can be either an absolute path or relative to the directory that contains your config. |
 | **`imageSizes`**       | If specified, image uploads will be automatically resized in accordance to these image sizes. [More](#image-sizes) |
-| **`adminThumbnail`**   | Which of your provided image sizes to use for the admin panel's thumbnail. Typically a size around 500x500px or so works well. |
+| **`adminThumbnail`**   | Set the way that the Admin panel will display thumbnails for this Collection. [More](#admin-thumbnails) |
 
 *An asterisk denotes that a property above is required.*
 
@@ -111,6 +111,28 @@ The Payload Admin panel will also automatically display all available files, inc
 
 Behind the scenes, Payload relies on [`sharp`](https://sharp.pixelplumbing.com/api-resize#resize) to perform its image resizing. You can specify additional options for `sharp` to use while resizing your images.
 
+### Admin thumbnails
+
+You can specify how Payload retrieves admin thumbnails for your upload-enabled Collections. This property accepts two different configurations:
+
+1. A string equal to one of your provided image size names to use for the admin panel's thumbnail (seen in the example Media collection above)
+1. A function that takes the document's data and sends back a full URL to load the thumbnail. For example, to dynamically set an admin thumbnail URL, you could write a function that returns a string pointing to a different file source:
+
+**Example custom Admin thumbnail:**
+```js
+const Media = {
+  slug: 'media',
+  upload: {
+    staticURL: '/media',
+    staticDir: 'media',
+    imageSizes: [
+      // ... image sizes here
+    ],
+    adminThumbnail: ({ doc }) => `https://google.com/custom-path-to-file/${doc.filename}`,
+  }
+}
+```
+
 ### Uploading Files
 
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