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.