ndarray `size()` needs a better docstring or just not exist

https://github.com/quantopian/libpy/blob/master/include/libpy/ndarray_view.h#L804. 

The following should always work:

```cpp
for (std::size_t ix = 0; ix < arr.size(); ++ix) {
    arr[ix];
}
```

Docstring should be `the size along axis 0` or just not exist. 