Only update class docstring if ipython is available

Currently param updates class docstrings to help with tab completion. I think this behavior is useful (and can be disabled by setting the `docstring_signature` variable to `False` but it might be worth considering two tweaks to this behavior:

1) Only doing this for `ParameterizedFunction` (might be tricky, currently the logic is on `ParameterizedMetaclass`)
2) Only doing this when in an ipython context. This may also be hacky as it might mean trying to detect `get_ipython` in the namespace (or similar).

I just want to consider these options as I would prefer to mutate docstrings *only* when tab-completion is actually relevant.