[docs] Document PySequence_List(o) as equivalent to list(o) (issue 23081)

[berker.peksag at gmail.com]

http://bugs.python.org/review/23081/diff/13453/Doc/c-api/sequence.rst
File Doc/c-api/sequence.rst (right):

http://bugs.python.org/review/23081/diff/13453/Doc/c-api/sequence.rst#newcode111
Doc/c-api/sequence.rst:111: or *NULL* on failure.  The returned list is
guaranteed to be new.  This is
*NULL* -> ``NULL``

http://bugs.python.org/review/23081/