Merge pull request #72161 from openshift-cherrypick-robot/cherry-pick-72092-to-enterprise-4.16

[enterprise-4.16] Adding outpost subnet requirement to VPC install doc

modules/installation-custom-aws-vpc.adoc

@@ -99,6 +99,7 @@ endif::aws-outposts[]
 * The VPC must not use the `kubernetes.io/cluster/.*: owned`, `Name`, and `openshift.io/cluster` tags.
 +
 The installation program modifies your subnets to add the `kubernetes.io/cluster/.*: shared` tag, so your subnets must have at least one free tag slot available for it. See link:https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Using_Tags.html#tag-restrictions[Tag Restrictions] in the AWS documentation to confirm that the installation program can add a tag to each subnet that you specify. You cannot use a `Name` tag, because it overlaps with the EC2 `Name` field and the installation fails.
+* If you want to extend your {product-title} cluster into an AWS Outpost and have an existing Outpost subnet, the existing subnet must use the `kubernetes.io/cluster/unmanaged: true` tag. If you do not apply this tag, the installation might fail due to the Cloud Controller Manager creating a service load balancer in the Outpost subnet, which is an unsupported configuration.
 * You must enable the `enableDnsSupport` and `enableDnsHostnames` attributes in your VPC, so that the cluster can use the Route 53 zones that are attached to the VPC to resolve cluster's internal DNS records. See link:https://docs.aws.amazon.com/vpc/latest/userguide/vpc-dns.html#vpc-dns-support[DNS Support in Your VPC] in the AWS documentation.
 +
 If you prefer to use your own Route 53 hosted private zone, you must associate the existing hosted zone with your VPC prior to installing a cluster. You can define your hosted zone using the `platform.aws.hostedZone` and `platform.aws.hostedZoneRole` fields in the `install-config.yaml` file.