Describe the default_namespace parameter of ElemetTree.write.

diff --cc Doc/library/xml.etree.elementtree.rst
index 26f1fbe7a17858e4514634f9643098c4deac76d6,d9b0a520ef3a6ce5a8c1e82a05c0e1d364fc027e..144e344397abd5fdba382a795f81ad460924822c
--- 1/Doc/library/xml.etree.elementtree.rst
--- 2/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@@ -746,20 -473,13 +746,21 @@@ ElementTree Object
  
        Writes the element tree to a file, as XML.  *file* is a file name, or a
        :term:`file object` opened for writing.  *encoding* [1]_ is the output
 -      encoding (default is US-ASCII).  Use ``encoding="unicode"`` to write a
 -      Unicode string.  *xml_declaration* controls if an XML declaration
 -      should be added to the file.  Use False for never, True for always, None
 -      for only if not US-ASCII or UTF-8 or Unicode (default is None).
 +      encoding (default is US-ASCII).
 +      *xml_declaration* controls if an XML declaration should be added to the
 +      file.  Use ``False`` for never, ``True`` for always, ``None``
 +      for only if not US-ASCII or UTF-8 or Unicode (default is ``None``).
+       *default_namespace* sets the default XML namespace (for "xmlns").
        *method* is either ``"xml"``, ``"html"`` or ``"text"`` (default is
 -      ``"xml"``).  Returns an (optionally) encoded string.
 +      ``"xml"``).
 +
 +      The output is either a string (:class:`str`) or binary (:class:`bytes`).
 +      This is controlled by the *encoding* argument.  If *encoding* is
 +      ``"unicode"``, the output is a string; otherwise, it's binary.  Note that
 +      this may conflict with the type of *file* if it's an open
 +      :term:`file object`; make sure you do not try to write a string to a
 +      binary stream and vice versa.
 +
  
  This is the XML file that is going to be manipulated::
  

diff --cc Lib/xml/etree/ElementTree.py
index 9553c51f6c56636893bbff40debf83b577668e4e,ff8ff7d78f4e241e7987869a886f96b74904e692..e8e309c3a0436e076fdb70332f3b5a27d6a498be
--- 1/Lib/xml/etree/ElementTree.py
--- 2/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@@ -792,10 -805,12 +792,11 @@@ class ElementTree
      # @keyparam xml_declaration Controls if an XML declaration should
      #     be added to the file.  Use False for never, True for always,
      #     None for only if not US-ASCII or UTF-8 or Unicode.  None is default.
+     # @keyparam default_namespace Sets the default XML namespace (for "xmlns").
+     # @keyparam method Optional output method ("xml", "html", "text" or
+     #     "c14n"; default is "xml").
  
      def write(self, file_or_filename,
 -              # keyword arguments
                encoding=None,
                xml_declaration=None,
                default_namespace=None,