Can AppSync owner field be used as primary key?

The [GraphQL Per-user / owner-based data access](https://docs.amplify.aws/cli/graphql/authorization-rules/#per-user--owner-based-data-access) docs state the following:

> Do not set ownerField to your @primaryKey field or id field if no primary key is specified. If you want to query by the ownerField, use an @index on that ownerField to create a secondary index.

I'm seeking clarification around if/when it is appropriate to apply the @primaryKey directive to the owner field. My ultimate goal is to enforce a 1:1 constraint for a given Cognito user and GraphQL model.

Start with the linked GraphQL per-user/owner-based data access documentation, especially the guidance for ownerField, @primaryKey, and @index. Determine whether using the owner field as a primary key is supported for the stated 1:1 Cognito-user constraint, then update the documentation with the applicable clarification and examples.

Written by the indexing model from the issue text.