docs: Add Federated Principal information to iam_policy_document
docs: Add Federated Principal information to iam_policy_document

This data source already supports Federated access. It just isn't
documented. This change adds that documentation as well as providing
an example of it being used.

kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws