Don't use state anywhere for the whole state tree

[mcabbott]

Doc-only change.

We use state for what init returns, so should probably never use it for what setup returns. That's called tree in code, and this calls it state_tree more places in the docs.

Unsure if we should change headings like Optimisers.update!(tree, model, gradient) -> (tree, model) to the more verbose Optimisers.update!(state_tree, model, gradient) -> (state_tree, model).

Another possibility for docs is to call the whole thing states plural.