[docs] Document that datetime.__format__ is datetime.strftime (issue 8913)

[ezio.melotti at gmail.com]

http://bugs.python.org/review/8913/diff/6442/Doc/library/datetime.rst
File Doc/library/datetime.rst (right):

http://bugs.python.org/review/8913/diff/6442/Doc/library/datetime.rst#newcode616
Doc/library/datetime.rst:616: 'John in 2002'
I'm not sure what 'John in 2002' means.
I would re-use one of the previous examples to show the symmetry between
strftime and format.

http://bugs.python.org/review/8913/