[Docs+feature] Consider how Var should be represented in `reveal_type` and hover

[stroxler]

Describe the Bug

We already show @_ on hover if a type is never pinned; I'm in the process of changing reveal_type to do the same if it is not yet pinned when the reveal_type is analyzed.

We need documentation for what this means, and we might want to also change the printable representation to something more informative (and more searchable in our docs page)

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response