vimdoc can not generate doc for var like `g:settings.default_indent`

I have this viml script.
```viml
    ""
    " The default_indent of SpaceVim.
    let g:settings.default_indent          = 2

```

but vimdoc just generate:
```
                                                                  *g:settings*
The default_indent of SpaceVim.
```
```