Documents are made up of block nodes, inline nodes, and text nodes—just like in the DOM.
In some places, you'll see mention of "fragments", which are also `Document` objects, just that aren't attached to the main `State`. For example, when cutting-and-pasting a selection of content, that content will be referred to as a document "fragment".
An immutable string value of `'document'` for easily separating this node from [`Block`](./block.md), [`Inline`](./inline.md) or [`Text`](./text.md) nodes.