Document the one-to-one non-nullable relationship limitation

[lastmjs]

Right now you cannot have a one-to-one relationship where both of the fields are non-nullable, because one record must be created before the other, otherwise you will never satisfy the requirements of the GraphQL query.

This should be possible once we enable nested creates, because you could always create the other record while creating the other record. We should probably wait until we have indexing, benchmarks, automatic migrations, and authorization before we move onto nested creates