doc update for subnet auto-discovery (kubernetes-sigs#2374)

docs/deploy/subnet_discovery.md

@@ -1,9 +1,11 @@
 # Subnet Auto Discovery
 AWS Load Balancer controller auto discovers network subnets for ALB or NLB by default. ALB requires at least two subnets across Availability Zones, NLB requires one subnet.
-The subnets must be tagged appropriately for the auto discovery to work. The controller chooses one subnet from each Availability Zone. In case of multiple tagged subnets in
-an Availability Zone, the controller will choose the first one in lexicographical order by the Subnet IDs. If you use `eksctl` or an Amazon EKS AWS CloudFormation template to
- create your VPC after March 26, 2020, then the subnets are tagged appropriately when they're created. For more information about the Amazon EKS AWS CloudFormation VPC templates,
- see [Creating a VPC for your Amazon EKS cluster](https://docs.aws.amazon.com/eks/latest/userguide/create-public-private-vpc.html).
+The subnets must be tagged appropriately for the auto discovery to work. The controller chooses one subnet from each Availability Zone. During auto-discovery, the controller
+considers subnets with at lease 8 available ip addresses. In case of multiple qualified tagged subnets in an Availability Zone, the controller chooses the first one in lexicographical 
+order by the Subnet IDs. For more information about the subnets for the AWS load balancer, see [Application Load Balancers](https://docs.aws.amazon.com/elasticloadbalancing/latest/application/application-load-balancers.html) 
+and [Network Load Balancers](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/network-load-balancers.html).
+If you use `eksctl` or an Amazon EKS AWS CloudFormation template to create your VPC after March 26, 2020, then the subnets are tagged appropriately when they're created. For 
+more information about the Amazon EKS AWS CloudFormation VPC templates, see [Creating a VPC for your Amazon EKS cluster](https://docs.aws.amazon.com/eks/latest/userguide/create-public-private-vpc.html).
 
 ## Public subnets
 Public subnets are used for internet-facing load balancers. These subnets must have the following tags: