Add docs and changelog

Author: alexanderldavis

.changelog/44560.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+resource/aws_vpclattice_resource_gateway: Add `ipv4_addresses_per_eni` argument to `aws_vpclattice_resource_gateway`
+```

website/docs/r/vpclattice_resource_gateway.html.markdown

@@ -63,6 +63,7 @@ The following arguments are optional:
 
 * `region` - (Optional) Region where this resource will be [managed](https://docs.aws.amazon.com/general/latest/gr/rande.html#regional-endpoints). Defaults to the Region set in the [provider configuration](https://registry.terraform.io/providers/hashicorp/aws/latest/docs#aws-configuration-reference).
 * `ip_address_type` - (Optional) IP address type used by the resource gateway. Valid values are `IPV4`, `IPV6`, and `DUALSTACK`. The IP address type of a resource gateway must be compatible with the subnets of the resource gateway and the IP address type of the resource.
+* `ipv4_addresses_per_eni` - (Optional) The number of IPv4 addresses per ENI for your resource. This argument is only applicable to `IPV4` and `DUALSTACK` IP address types. Defaults to `16`.
 * `security_group_ids` - (Optional) Security group IDs associated with the resource gateway. The security groups must be in the same VPC.
 * `tags` - (Optional) Key-value mapping of resource tags. If configured with a provider [`default_tags` configuration block](/docs/providers/aws/index.html#default_tags-configuration-block) present, tags with matching keys will overwrite those defined at the provider-level.