Update the docstring.

Add a Node class that defines the NodeType constants, based on discussion
in the XML-SIG.

Lib/xml/dom/__init__.py

 """W3C Document Object Model implementation for Python.
 
-The Python mapping of the Document Object Model is documented in <...>.
+The Python mapping of the Document Object Model is documented in the
+Python Library Reference in the section on the xml.dom package.
 
 This package contains the following modules:
 
 minidom -- A simple implementation of the Level 1 DOM with namespace
-           support added (based on the Level 2 specification).
+           support added (based on the Level 2 specification) and other
+           minor Level 2 functionality.
+
+pulldom -- DOM builder supporting on-demand tree-building for selected
+           subtrees of the document.
 
 """
+
+
+class Node:
+    """Class giving the NodeType constants."""
+
+    # DOM implementations may use this as a base class for their own
+    # Node implementations.  If they don't, the constants defined here
+    # should still be used as the canonical definitions as they match
+    # the values given in the W3C recommendation.  Client code can
+    # safely refer to these values in all tests of Node.nodeType
+    # values.
+
+    ELEMENT_NODE                = 1
+    ATTRIBUTE_NODE              = 2
+    TEXT_NODE                   = 3
+    CDATA_SECTION_NODE          = 4
+    ENTITY_REFERENCE_NODE       = 5
+    ENTITY_NODE                 = 6
+    PROCESSING_INSTRUCTION_NODE = 7
+    COMMENT_NODE                = 8
+    DOCUMENT_NODE               = 9
+    DOCUMENT_TYPE_NODE          = 10
+    DOCUMENT_FRAGMENT_NODE      = 11
+    NOTATION_NODE               = 12