bug: policy exception fieldkey should list valid accepted values.

[sbeaulie]

Describe the bug
Terraform documentation improvement, fieldkey should list valid accepted values. https://registry.terraform.io/providers/lacework/lacework/latest/docs/resources/policy_exception#field_key

To Reproduce
https://registry.terraform.io/providers/lacework/lacework/latest/docs/resources/policy_exception#field_key only list accountIds

Expected behavior
A list of valid values that can be used