Add documentation description for feature gate ALBSingleSubnet

Author: jerryhe1999

docs/deploy/configurations.md

@@ -172,3 +172,4 @@ They are a set of kye=value pairs that describe AWS load balance controller feat
 | EnableRGTAPI                       | string                          | false          | If enabled, the tagging manager will describe resource tags via RGT APIs, otherwise via ELB APIs. In order to enable RGT API, `tag:GetResources` is needed in controller IAM policy. |
 | SubnetsClusterTagCheck                | string                          | true           | Enable or disable the check for `kubernetes.io/cluster/${cluster-name}` during subnet auto-discovery |
 | NLBHealthCheckAdvancedConfiguration   | string                          | true           | Enable or disable advanced health check configuration for NLB, for example health check timeout |
+| ALBSingleSubnet                       | string                          | false          | If enabled, controller will allow using only 1 subnet for provisioning ALB, which need to get whitelisted by ELB in advance |

docs/deploy/subnet_discovery.md

@@ -1,5 +1,6 @@
 # Subnet auto-discovery
-By default, the AWS Load Balancer Controller (LBC) auto-discovers network subnets that it can create AWS Network Load Balancers (NLB) and AWS Application Load Balancers (ALB) in. ALBs require at least two subnets across Availability Zones. NLBs require one subnet.
+By default, the AWS Load Balancer Controller (LBC) auto-discovers network subnets that it can create AWS Network Load Balancers (NLB) and AWS Application Load Balancers (ALB) in. ALBs require at least two subnets across Availability Zones by default, 
+set [Feature Gate ALBSingleSubnet](https://kubernetes-sigs.github.io/aws-load-balancer-controller/latest/deploy/configurations/#feature-gates) to "true" allows using only 1 subnet for provisioning ALB. NLBs require one subnet.
 The subnets must be tagged appropriately for auto-discovery to work. The controller chooses one subnet from each Availability Zone. During auto-discovery, the controller
 considers subnets with at least eight available IP addresses. In the case of multiple qualified tagged subnets in an Availability Zone, the controller chooses the first one in lexicographical 
 order by the subnet IDs.

pkg/networking/subnet_resolver.go

@@ -48,7 +48,7 @@ type SubnetsResolveOptions struct {
 	AvailableIPAddressCount int64
 	// whether to check the cluster tag
 	SubnetsClusterTagCheck bool
-	// Disable subnet minimal count restriction
+	// whether to allow using only 1 subnet for provisioning ALB, default to false
 	ALBSingleSubnet bool
 }
 
@@ -97,7 +97,7 @@ func WithSubnetsClusterTagCheck(SubnetsClusterTagCheck bool) SubnetsResolveOptio
 	}
 }
 
-// WithALBSingleSubnet generate an option that foncigure ALBSingleSubnet
+// WithALBSingleSubnet generate an option that configures ALBSingleSubnet
 func WithALBSingleSubnet(ALBSingleSubnet bool) SubnetsResolveOption {
 	return func(opts *SubnetsResolveOptions) {
 		opts.ALBSingleSubnet = ALBSingleSubnet