Document what can be the element name, the attributes and the text

They are strings or QName instances, and the text and the tail can also be
None.  The element name can also be Comment, ProcessingInstruction or None,
and the attribute value can be None for the HTML method.

Author: serhiy-storchaka

Doc/library/xml.etree.elementtree.rst

@@ -891,6 +891,18 @@ Element Objects
    an optional dictionary, containing element attributes.  *extra* contains
    additional attributes, given as keyword arguments.
 
+   The element name and the attribute names and values are strings or
+   :class:`QName` instances, and the text and the tail are strings or
+   ``None``.
+   The element name can also be :func:`Comment` or
+   :func:`ProcessingInstruction`, which are used for special elements.
+   If it is ``None``, the element itself is not serialized: only its text
+   and its children are written, and its attributes are ignored.
+   This can be used for a fragment which contains several elements.
+   With ``method="html"`` the attribute value can also be ``None``,
+   which produces an empty attribute (such as ``checked``).
+   Other objects can be stored in the tree, but they cannot be serialized.
+
    .. versionchanged:: 3.15
       *attrib* can now be a :class:`frozendict`.