docs: clarify file upload example with _payload & field explanation (#12025)

### What?

This PR updates the `Uploading Files` section in the `Uploads` docs to:

- Use `_payload` in the file upload example, which is required for
non-file fields to be parsed correctly by Payload.
- Add a clear comment explaining that the fields inside `_payload`
should match the schema of the upload-enabled collection.

### Why?

These changes aim to reduce confusion when uploading files via the REST
API.

Fixes #11681

docs/admin/locked-documents.mdx

@@ -21,10 +21,9 @@ When a user starts editing a document, Payload locks it for that user. If anothe
 The lock will automatically expire after a set period of inactivity, configurable using the `duration` property in the `lockDocuments` configuration, after which others can resume editing.
 
 <Banner type="info">
-  {' '}
   **Note:** If your application does not require document locking, you can
   disable this feature for any collection or global by setting the
-  `lockDocuments` property to `false`.{' '}
+  `lockDocuments` property to `false`.
 </Banner>
 
 ### Config Options

docs/upload/overview.mdx

@@ -334,12 +334,28 @@ To upload a file, use your collection's [`create`](/docs/rest-api/overview#colle
 
 Send your request as a `multipart/form-data` request, using [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) if possible.
 
+<Banner type="info">
+  **Note:** To include any additional fields (like `title`, `alt`, etc.), append
+  a `_payload` field containing a JSON-stringified object of the required
+  values. These values must match the schema of your upload-enabled collection.
+</Banner>
+
 ```ts
 const fileInput = document.querySelector('#your-file-input')
 const formData = new FormData()
 
 formData.append('file', fileInput.files[0])
 
+// Replace with the fields defined in your upload-enabled collection.
+// The example below includes an optional field like 'title'.
+formData.append(
+  '_payload',
+  JSON.stringify({
+    title: 'Example Title',
+    description: 'An optional description for the file',
+  }),
+)
+
 fetch('api/:upload-slug', {
   method: 'POST',
   body: formData,