showdoc only shows first returned object for numpy docstrings

For a function that returns multiple objects, parsing a numpy-style doctring with nbdev.showdoc (and nbdev_preview in the command window) only shows the first returned object. Example:

```
def f(x):
"""
foo

Parameters
----------
x : int

Returns
-------
a : int
b : str

"""
return a, b
```

This gives a docstring that only mentions "a" and not "b".

Start by tracing how nbdev.showdoc and nbdev_preview parse the NumPy-style Returns section, using the provided function as a reproduction case. Done means the generated documentation mentions both returned objects, “a” and “b,” rather than only the first.

Written by the indexing model from the issue text.