chore(docs): add note about subnet AZ uniqueness requirement (#3039)

Author: jpalomaki

Diff for: README.md

@@ -535,7 +535,7 @@ We welcome any improvement to the standard module to make the default as secure
 | <a name="input_scale_down_schedule_expression"></a> [scale\_down\_schedule\_expression](#input\_scale\_down\_schedule\_expression) | Scheduler expression to check every x for scale down. | `string` | `"cron(*/5 * * * ? *)"` | no |
 | <a name="input_scale_up_reserved_concurrent_executions"></a> [scale\_up\_reserved\_concurrent\_executions](#input\_scale\_up\_reserved\_concurrent\_executions) | Amount of reserved concurrent executions for the scale-up lambda function. A value of 0 disables lambda from being triggered and -1 removes any concurrency limitations. | `number` | `1` | no |
 | <a name="input_ssm_paths"></a> [ssm\_paths](#input\_ssm\_paths) | The root path used in SSM to store configuration and secreets. | <pre>object({<br>    root       = optional(string, "github-action-runners")<br>    app        = optional(string, "app")<br>    runners    = optional(string, "runners")<br>    use_prefix = optional(bool, true)<br>  })</pre> | `{}` | no |
-| <a name="input_subnet_ids"></a> [subnet\_ids](#input\_subnet\_ids) | List of subnets in which the action runners will be launched, the subnets needs to be subnets in the `vpc_id`. | `list(string)` | n/a | yes |
+| <a name="input_subnet_ids"></a> [subnet\_ids](#input\_subnet\_ids) | List of subnets in which the action runner instances will be launched. The subnets need to exist in the configured VPC (`vpc_id`), and must reside on different availability zones (see https://github.com/philips-labs/terraform-aws-github-runner/issues/2904) | `list(string)` | n/a | yes |
 | <a name="input_syncer_lambda_s3_key"></a> [syncer\_lambda\_s3\_key](#input\_syncer\_lambda\_s3\_key) | S3 key for syncer lambda function. Required if using S3 bucket to specify lambdas. | `string` | `null` | no |
 | <a name="input_syncer_lambda_s3_object_version"></a> [syncer\_lambda\_s3\_object\_version](#input\_syncer\_lambda\_s3\_object\_version) | S3 object version for syncer lambda function. Useful if S3 versioning is enabled on source bucket. | `string` | `null` | no |
 | <a name="input_tags"></a> [tags](#input\_tags) | Map of tags that will be added to created resources. By default resources will be tagged with name and environment. | `map(string)` | `{}` | no |

Diff for: variables.tf

@@ -9,7 +9,7 @@ variable "vpc_id" {
 }
 
 variable "subnet_ids" {
-  description = "List of subnets in which the action runners will be launched, the subnets needs to be subnets in the `vpc_id`."
+  description = "List of subnets in which the action runner instances will be launched. The subnets need to exist in the configured VPC (`vpc_id`), and must reside on different availability zones (see https://github.com/philips-labs/terraform-aws-github-runner/issues/2904)"
   type        = list(string)
 }