 NumpyDocString doesn't roundtrip well

```python
from numpydoc.tests.test_docscrape import doc_txt
from numpydoc import docscrape
docscrape.NumpyDocString(
    str(docscrape.NumpyDocString(doc_txt)))
```
leads to

```
ParseError: `some`_ is not a item name in 'numpy.multivariate_normal(m 
[...]
```

This would be really useful if we want to implement merging (or adding) NumpyDocStrings