Docs: clarify DocumentType identifiers

Author: Neiland85

Doc/library/xml.dom.rst

@@ -453,14 +453,24 @@ following attributes:
 
 .. attribute:: DocumentType.publicId
 
-   The public identifier for the external subset of the document type definition.
-   This will be a string or ``None``.
+   The public identifier for the external subset of the document type definition,
+   or ``None`` if the ``DOCTYPE`` declaration does not specify one. A ``PUBLIC``
+   ``DOCTYPE`` declaration always has both a public and a system identifier.
 
 
 .. attribute:: DocumentType.systemId
 
-   The system identifier for the external subset of the document type definition.
-   This will be a URI as a string, or ``None``.
+   The system identifier for the external subset of the document type definition,
+   or ``None`` if the ``DOCTYPE`` declaration does not specify one. A ``SYSTEM``
+   ``DOCTYPE`` declaration has a system identifier but no public identifier.
+   The system identifier is a URI as a string.
+
+   For a parsed XML document, ``publicId`` and ``systemId`` correspond to the
+   external identifier in the ``DOCTYPE`` declaration: both are non-``None`` for
+   a ``PUBLIC`` declaration, only ``systemId`` is non-``None`` for a ``SYSTEM``
+   declaration, and both are ``None`` when there is no external identifier.
+   :meth:`DOMImplementation.createDocumentType` does not validate the identifiers,
+   so programmatically created objects may have other combinations.
 
 
 .. attribute:: DocumentType.internalSubset