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

[heikki.partanen 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'
The idea there is to emphasize the handiness of being able to format a
date(time) within a single str.format() call. If there are no other
parameters in the example, this advantage may go unnoticed. 

It was actually suggested earlier by Éric:
http://bugs.python.org/review/8913/diff/6363/Doc/library/datetime.rst

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