List widget with variable types should allow hints or descriptions for each type

I'm making extensive use of variable types in the list widget, and it has been extremely useful! One thing I would like to see would be the ability to add a hint or description for the whole type. My current work around is adding a dummy boolean widget and giving it a hint, but that feels kind of hacky. The ideal solution would be to support a hint or description for the entire type that would show at the top of the control for each type.

For example:
```
label: 'Home Section'
name: 'sections'
widget: 'list'
types:
- label: 'Carousel'
name: 'carousel'
widget: object
hint: 'this is a description of the carousel type. This description should render above any fields for this type",
fields:
- { label: Header, name: header, widget: string, default: 'Image Gallery' }
...
```

Another possibly easier change would be to allow hidden fields to display hints if they have any.