Docs for `extendedWhereUnique` feature

[andrew-walford-prisma]

Add draft documentation for new extendedWhereUnique feature

[vercel[bot]]

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Name	Status	Preview	Updated
docs	✅ Ready (Inspect)	Visit Preview	Oct 18, 2022 at 7:42AM (UTC)

[github-actions[bot]]

This PR changes the following pages (Vercel Preview Deploy links):

Changed Vercel links

• /docs/concepts/components/prisma-client/middleware/soft-delete-middleware (currently)
• /docs/concepts/components/preview-features/client-preview-features (currently)
• /docs/guides/performance-and-optimization/prisma-client-transactions-guide (currently)
• /docs/reference/api-reference/prisma-client-reference (currently)

(Note that links will only be valid after Vercel preview deploy succeeded)