Add detailed documentation for Part.Shape

[Fraktality]

Changes

• Make the table consistent with https://create.roblox.com/docs/reference/engine/enums/PartType
• Add details about exactness guarantees for part collisions

Checks

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• To the best of my knowledge, all proposed changes are accurate.

[github-actions]

Hi @Fraktality, thanks so much for helping improve the Roblox creator documentation! Our technical writing team will review your pull request soon. In the meantime, please ensure you've read through the README.md, contribution guidelines, and style recommendations.