docs: CRUD mutations are not generated for polymorphic types

[FelixZY]

Description

This was mentioned by @benjie in #2077 and #2083 but does not appear to have been explicitly documented yet.

These changes were not discussed beforehand so do let me know if these docs should not be added or are added to the wrong place.

Performance impact

N/A

Security impact

N/A

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[changeset-bot]

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Latest commit: b443c26

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[benjie]

Thanks! 🙌