Rename `referentialIntegrity` field to `relationMode`

[keerlu]

Describe this PR

• Rename the referentialIntegrity field in the datasource block to relationMode
• Rework the text to fit

Changes

What issue does this fix?

Fixes #3837

Any other relevant information

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
docs	✅ Ready (Inspect)	Visit Preview	Oct 18, 2022 at 5:30PM (UTC)

[github-actions[bot]]

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

Changed Vercel links

• /docs/getting-started/setup-prisma/start-from-scratch/relational-databases/connect-your-database (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases/connect-your-database (currently)
• /docs/concepts/components/prisma-schema/relations/referential-actions/special-rules-for-referential-actions (currently)
• /docs/concepts/components/prisma-schema/relations/referential-actions (currently)
• /docs/concepts/components/prisma-schema/relations/referential-integrity (currently)
• /docs/guides/database/using-prisma-with-planetscale (currently)
• /docs/reference/api-reference/prisma-schema-reference (currently)

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