[red-knot] rename Definition::None to Definition::Unbound (#11758)
## Summary
After looking at this a bit, I think it does make sense to have
`Unbound` as part of the `Definition` enum; if we are modeling `Unbound`
as a type (which currently we are), then every symbol implicitly starts
each scope with a "definition" as unbound, and the cleanest way to model
that is as a real `Definition`. We should be able to handle a definition
of "unbound" anywhere we handle definitions.
But the name `None` wasn't clear enough; changing the name to `Unbound`
and adding a doc comment.
Also change `[first].into_iter()` to `std::iter::once(first)`, from
post-land code review on a prior PR.
## Test Plan
Existing tests.
