Move DirectoryTree.FileSelected.path's doc to inline style

This brings it in line with the documentation for other widgets and their
messages.

src/textual/widgets/_directory_tree.py

@@ -73,14 +73,12 @@ class DirectoryTree(Tree[DirEntry]):
 
         Can be handled using `on_directory_tree_file_selected` in a subclass of
         `DirectoryTree` or in a parent widget in the DOM.
-
-        Attributes:
-            path: The path of the file that was selected.
         """
 
         def __init__(self, path: str) -> None:
-            self.path: str = path
             super().__init__()
+            self.path: str = path
+            """The path of the file that was selected."""
 
     def __init__(
         self,