feat: [PL-57533]: Updated documentation for Role assignment resource (#1081)

Author: niketan-nath

docs/resources/platform_role_assignments.md

@@ -14,6 +14,8 @@ References:
 - For details on how to onboard with Terraform, please see [Harness Terraform Provider Overview](https://developer.harness.io/docs/platform/automation/terraform/harness-terraform-provider-overview/)
 - To understand how to use Role Assignments, please see [Documentation](https://developer.harness.io/docs/platform/Role-Based-Access-Control/rbac-in-harness#role-assignment)
 - To get more information about Api, please see [API documentation](https://apidocs.harness.io/tag/Role-Assignments)
+
+Note: Role assignments for principal type `SERVICE` and `API_KEY` cannot be changed.
 ## Example Usage
 
 ```terraform