Update docstring for FileUpload class

Clarified the docstring to specify usage within Label class.

Author: Sacul0457

discord/ui/file_upload.py

@@ -53,7 +53,7 @@
 
 
 class FileUpload(Item[V]):
-    """Represents a file upload component within a modal.
+    """Represents a file upload component within a modal that can only be used in :class:`Label`.
 
     .. versionadded:: 2.7