[docs] Update documented signatures of tp_get/setattr (issue 28771)

[storchaka+cpython at gmail.com]

http://bugs.python.org/review/28771/diff/19257/Doc/c-api/typeobj.rst
File Doc/c-api/typeobj.rst (right):

http://bugs.python.org/review/28771/diff/19257/Doc/c-api/typeobj.rst#newcode204
Doc/c-api/typeobj.rst:204: PyObject * tp_getattr(PyObject *o, char
*attr_name);
Shouldn't we use the ".. code-block:: c" directive?

Please document explicitly that attr_name point't to read-only string,
its content shouldn't be modified.

http://bugs.python.org/review/28771/diff/19257/Doc/c-api/typeobj.rst#newcode219
Doc/c-api/typeobj.rst:219: PyObject * tp_setattr(PyObject *o, char
*attr_name, PyObject *v);
Please document explicitly that "setting *v* to *NULL* to delete an
attribute must be supported".

http://bugs.python.org/review/28771/