docs: clarify security group name comes from tag

[bendrucker]

This clarifies the alb.ingress.kubernetes.io/security-groups documentation. We found ourselves momentarily confused when our security groups were not attached to the load balancer. I read the code and realized it's looking for a Name tag, but we'd only specified the groupName property (https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_SecurityGroup.html).

https://github.com/kubernetes-sigs/aws-alb-ingress-controller/blob/4bba2f2b0ba2fb171c38c9382a85627c196f48e9/internal/aws/ec2.go#L167-L189

The code later assumes that multiple values could be returned which is possible with tag:Name but not possible when setting the actual group name (must be unique). I found this behavior surprising (since the user can specify multiple SG names if they want to match multiple SGs).

If you're interested in supporting groupName, I'd be happy to work on a PR that makes two API calls in GetSecurityGroupsByName, one filtering by name tag and the other by group name, and then merges the results. In the mean time, wanted to make sure the docs clarified the current behavior appropriately.

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[bendrucker]

/assign @M00nF1sh

[M00nF1sh]

/ok-to-test
/lgtm

[M00nF1sh]

Thanks so much for making this change.
I think it's maybe better not to add a groupName discovery for now since it will add another APICalls on every reconcile, and discover by tag:name should be able to satisfy all use cases :D.(i'm open to this and happy to add support for groupName if there is customers for this :D)

[bendrucker]

Agreed, I think it's safe to assume that someone that needs this will consult the docs. If we could start from scratch I'd definitely make a case for groupName but the upside of changing things now is small.

[M00nF1sh]

/approve

[bendrucker]

Seems like prow didn't pick up the edited commit

[M00nF1sh]

/approve

[k8s-ci-robot]

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