chore: Simplify stable api docstrings

[MarcoGorelli]

The current check for the stable api docstrings is probably not worth the effort:

• we don't have stable.v1-specialised docstrings for everything
• it's annoying to have to redefine every top-level function in the stable api. We should only need to do so if:
  • we need to invoke _stableify
  • there is some breaking change we wish to preserve in v1
• simpler tests mean that the contribution process is simplified, and that's ultimately a top-priority goal

What type of PR is this? (check all applicable)

• [ ] 💾 Refactor
• [ ] ✨ Feature
• [ ] 🐛 Bug Fix
• [ ] 🔧 Optimization
• [ ] 📝 Documentation
• [ ] ✅ Test
• [ ] 🐳 Other

Related issues

• Related issue #
• Closes #

Checklist

• [ ] Code follows style guide (ruff)
• [ ] Tests added
• [ ] Documented the changes

If you have comments or can explain your changes, please do so below.