Clarify @updatedAt behavior in documentation

[panzi]

Problem

If there are no other columns to update, then the column marked with @updatedAt is not changed, and the update/upsert call doesn't even access the database.
This behavior is not necessarily obvious and should be mentioned in the documentation. Such a documentation would have saved me the time to manually test the behavior just now (after I noticed a cache invalidation bug in my code because of this).

Suggested solution

Add this text:

If there are no other columns to update, then the column marked with @updatedAt is not changed, and the update/upsert call doesn't even access the database. If you want to ensure the column to be updated you need to manually set it in that case.

To this documentation: https://www.prisma.io/docs/reference/api-reference/prisma-schema-reference#updatedat

(Although I haven't checked the "and the update/upsert call doesn't even access the database" part.)