[docs] Improve cross-references in C API (issue 19195)

[georg at python.org]

http://bugs.python.org/review/19195/diff/9475/Doc/c-api/buffer.rst
File Doc/c-api/buffer.rst (right):

http://bugs.python.org/review/19195/diff/9475/Doc/c-api/buffer.rst#newcode47
Doc/c-api/buffer.rst:47: the buffer interface can be written to a file. 
While :meth:`~io.BufferedIOBase.write` only
just replace by ``write()``, two references in two sentences is not
needed

http://bugs.python.org/review/19195/diff/9475/Doc/c-api/typeobj.rst
File Doc/c-api/typeobj.rst (right):

http://bugs.python.org/review/19195/diff/9475/Doc/c-api/typeobj.rst#newcode643
Doc/c-api/typeobj.rst:643: When a type's :attr:`~object.__slots__`
declaration contains a slot named
here too: too many links are not needed and distract.

http://bugs.python.org/review/19195/