Here is a patch. I'm not familiar with this part of the CPython source. So please tell me if there is a better way to do it. I only updated Doc/whatsnew/3.5, but other places in the documentation needs to be updated.
If make docstrings writable, it would be good to ensure that they exactly are strings. And if make the property doctstring writable, may be make other docstrings writable? It may be useful for setting the same docstring for Python implementation and C accelerator.