DOC: documenting cython class (eg Timestamp): "cyfunction is not a python function"
Follows from discussion in PR  #5160.

When adding `Timestamp` in api.rst to document this class and its methods (as eg done in this commit https://github.com/jorisvandenbossche/pandas/commit/3a8a41dda11d47d337e3f916567e8af2772598c2), you get warnings when building the docs like for example:

```
WARNING: error while formatting arguments for pandas.Timestamp.to_period: 
<cyfunction Timestamp.to_period at 0x00000000057616C0> is not a Python 
function.
```

`Timestamp` is a python class in a cython file (so I suppose then it is a cython class). You get the warnings for all methods that are defined in this class (defined here https://github.com/pydata/pandas/blob/master/pandas/tslib.pyx#L130), but not for inherited functions of _Timestamp or datetime.

It seems that the generated docstring pages are built (so with the docstring rendered), but that Sphinx could not detect the signature. 

Does somebody know how to let Sphinx handle cython classes/functions?
