_str_members_list mishandles multi-line descriptions

http://scikit-learn.org/dev/modules/generated/sklearn.model_selection.GridSearchCV.html under "Attributes" shows an example where a table is included in the attribute description. But `_str_members_list` destroys any layout [here](https://github.com/numpy/numpydoc/blob/7d7f11c/numpydoc/docscrape_sphinx.py#L151).

Does anyone know why multiple lines of description are flattened?