[docs] ElementTree: Document handling processing instructions (issue 20375)

[pcmanticore at gmail.com]

http://bugs.python.org/review/20375/diff/10762/Lib/test/test_xml_etree.py
File Lib/test/test_xml_etree.py (right):

http://bugs.python.org/review/20375/diff/10762/Lib/test/test_xml_etree.py#newcode1628
Lib/test/test_xml_etree.py:1628:
self.assertTrue(len(tree2.getroot().getchildren()) == 0)
Why not self.assertEqual(len(tree..), 0)?

http://bugs.python.org/review/20375/diff/10762/Lib/test/test_xml_etree.py#newcode1639
Lib/test/test_xml_etree.py:1639: self.assertTrue(b'<!--not to be
rendered-->' in bytes(stream.getbuffer()))
self.assertIn(b'..', bytes(..)) is clearer.

http://bugs.python.org/review/20375/diff/10762/Lib/test/test_xml_etree.py#newcode1644
Lib/test/test_xml_etree.py:1644:
self.assertTrue(len(tree2.getroot().getchildren()) == 0)
As above.

http://bugs.python.org/review/20375/