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