Document package.json generation in custom paths

[andrew-walford-prisma]

SME is Serhii

This has been pushed back. Also, the error is covered in the CLI, so it might not need docs as well.

The solution, if any, hasn't been decided on - it's pending more information.

Tech spec: https://www.notion.so/prismaio/package-json-generation-for-custom-output-paths-b264a29549434121978d0571305150fa