docs: add doc-strings for the doc-fields in `ContractType`

[antazoey]

What I did

both userdoc and devdoc were missing docs. I am currently trying to understand how NatSpec turns into these.

How I did it

How to verify it

Checklist

• [ ] Passes all linting checks (pre-commit and CI jobs)
• [ ] New test cases have been added and are passing
• [ ] Documentation has been updated
• [ ] PR title follows Conventional Commit standard (will be automatically included in the changelog)