gh-151943: Do not claim that nodeName can be None

The table above gives the value for every node type, and none of them
is None.

Author: serhiy-storchaka

Doc/library/xml.dom.rst

@@ -389,7 +389,6 @@ Inserting a child of a not allowed type raises :exc:`HierarchyRequestErr`.
    You can always get the information you would get here from another
    property such as the :attr:`~Element.tagName` property for elements or the
    :attr:`~Attr.name` property for attributes.
-   For all node types, the value of this attribute is a string or ``None``.
    This is a read-only attribute.