[docs] Provide some examples for usage of ElementTree methods/attributes (issue 15586)

[ezio.melotti at gmail.com]

http://bugs.python.org/review/15586/diff/5727/Doc/library/xml.etree.elementtree.rst
File Doc/library/xml.etree.elementtree.rst (right):

http://bugs.python.org/review/15586/diff/5727/Doc/library/xml.etree.elementtree.rst#newcode155
Doc/library/xml.etree.elementtree.rst:155: ... tree.write('output.xml')
This should now have the >>> prompt (same with the following examples).

http://bugs.python.org/review/15586/