bpo-33187: Document 3.9 changes to xml.etree.ElementInclude.include (GH-20438)

Looks like the merging of bpo-33187 and bpo-20928 was racy, resulting in
this change going undocumented.

Author: hauntsaninja

Doc/library/xml.etree.elementtree.rst

@@ -816,16 +816,25 @@ Functions
    loader fails, it can return None or raise an exception.
 
 
-.. function:: xml.etree.ElementInclude.include( elem, loader=None)
+.. function:: xml.etree.ElementInclude.include( elem, loader=None, base_url=None, \
+                                                max_depth=6)
 
    This function expands XInclude directives.  *elem* is the root element.  *loader* is
    an optional resource loader.  If omitted, it defaults to :func:`default_loader`.
    If given, it should be a callable that implements the same interface as
-   :func:`default_loader`.  Returns the expanded resource.  If the parse mode is
+   :func:`default_loader`.  *base_url* is base URL of the original file, to resolve
+   relative include file references.  *max_depth* is the maximum number of recursive
+   inclusions.  Limited to reduce the risk of malicious content explosion. Pass a
+   negative value to disable the limitation.
+
+   Returns the expanded resource.  If the parse mode is
    ``"xml"``, this is an ElementTree instance.  If the parse mode is "text",
    this is a Unicode string.  If the loader fails, it can return None or
    raise an exception.
 
+   .. versionadded:: 3.9
+      The *base_url* and *max_depth* parameters.
+
 
 .. _elementtree-element-objects: