Fix insert() signature

Author: AlexWaygood

stubs/docutils/docutils/nodes.pyi

@@ -109,7 +109,7 @@ class Element(Node):
     def astext(self) -> str: ...
     def index(self, item: Node, start: int = 0, stop: int = sys.maxsize) -> int: ...
     def remove(self, item: Node) -> None: ...
-    def insert(self, index: SupportsIndex, item: Node | Element | None) -> None: ...
+    def insert(self, index: SupportsIndex, item: Node | Iterable[Node] | None) -> None: ...
     def __getattr__(self, __name: str) -> Incomplete: ...
 
 class TextElement(Element):