This is intended to behave the same way as name_prefix does in aws_iam_role. Fixes #10176
This adds a note in the `aws_iam_policy_document` documentation that `resources` is required by AWS if used on an IAM policy. Also added a note on `aws_iam_policy` that `aws_iam_policy_document` is a good thing to use when configuring. Closes #9002