Paragraphs after the first in a list item should not be turned
into items of a separate list, and should round-trip:

```
% pandoc -f native -t docx -o - | pandoc -f docx -t native
[ BulletList
    [ [ Para [ Str "one" ]
      , Para [ Str "sub" ]
      , BulletList
          [ [ Para [ Str "nested" ]
            , Para [ Str "nested" , Space , Str "sub" ]
            ]
          ]
      , Para [ Str "back" ]
      ]
    , [ Para [ Str "two" ] ]
    ]
]
^D
[ BulletList
    [ [ Para [ Str "one" ]
      , Para [ Str "sub" ]
      , BulletList
          [ [ Para [ Str "nested" ]
            , Para [ Str "nested" , Space , Str "sub" ]
            ]
          ]
      , Para [ Str "back" ]
      ]
    , [ Para [ Str "two" ] ]
    ]
]
```