[ECS] Update compute_instance_v2 doc (#929)

[ECS] Update `compute_instance_v2` doc

Summary of the Pull Request
Update description of security_groups in resource/opentelekomcloud_compute_instance_v2
PR Checklist

 Refers to: #912
 Documentation updated.

Reviewed-by: Anton Kachurin <katchuring@gmail.com>
Reviewed-by: None <None>
Reviewed-by: Anton Sidelnikov <None>

docs/resources/compute_instance_v2.md

@@ -286,6 +286,9 @@ The following arguments are supported:
   to associate with the server. Changing this results in adding/removing
   security groups from the existing server.
 
+~> **Warning** Names should be used and not IDs. Security group names should be **unique**,
+otherwise it will return an error.
+
 -> When attaching the instance to networks using Ports,
   place the security groups on the Port and not the instance.