Add more documentation to special forms in `ty_extensions.pyi` stub

It's easy for users to jump to these definitions in the stub file from inlay hints. It will be even easier to jump to these definitions if something like https://github.com/astral-sh/ty/issues/1575 is implemented.

We should add more docstrings/comments to the stub file so that users have a better experience when they land there. Ideally we would also show this documentation on hover, so that users can easily learn about what the `Unknown` type means and represents, for example.

Start with the special-form definitions in ty_extensions.pyi and review the existing comments or docstrings. Add user-facing explanations for the definitions, including what the Unknown type means and represents; done means users landing on these definitions can understand them from the stub documentation.

Written by the indexing model from the issue text.