docs/resource/aws_instance: Clarify credit_specification configuratio…

…n block behaviors (hashicorp#17867)

Reference: hashicorp#17856

website/docs/r/instance.html.markdown

@@ -98,7 +98,7 @@ The following arguments are supported:
 
 * `cpu_core_count` - (Optional) Sets the number of CPU cores for an instance. This option is only supported on creation of instance type that support CPU Options [CPU Cores and Threads Per CPU Core Per Instance Type](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-optimize-cpu.html#cpu-options-supported-instances-values) - specifying this option for unsupported instance types will return an error from the EC2 API.
 * `cpu_threads_per_core` - (Optional - has no effect unless `cpu_core_count` is also set)  If set to to 1, hyperthreading is disabled on the launched instance. Defaults to 2 if not set. See [Optimizing CPU Options](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-optimize-cpu.html) for more information.
-* `credit_specification` - (Optional) Customize the credit specification of the instance. See [Credit Specification](#credit-specification) below for more details.
+* `credit_specification` - (Optional) Configuration block for customizing the credit specification of the instance. See [Credit Specification](#credit-specification) below for more details. Terraform will only perform drift detection of its value when present in a configuration. Removing this configuration on existing instances will only stop managing it. It will not change the configuration back to the default for the instance type.
 * `disable_api_termination` - (Optional) If true, enables [EC2 Instance Termination Protection](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/terminating-instances.html#Using_ChangingDisableAPITermination).
 * `ebs_block_device` - (Optional) One or more configuration blocks with additional EBS block devices to attach to the instance. Block device configurations only apply on resource creation. See [Block Devices](#ebs-ephemeral-and-root-block-devices) below for details on attributes and drift detection. When accessing this as an attribute reference, it is a set of objects.
 * `ebs_optimized` - (Optional) If true, the launched EC2 instance will be EBS-optimized. Note that if this is not set on an instance type that is optimized by default then this will show as disabled but if the instance type is optimized by default then there is no need to set this and there is no effect to disabling it. See the [EBS Optimized section](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSOptimized.html) of the AWS User Guide for more information.
@@ -146,10 +146,6 @@ The `timeouts` block allows you to specify [timeouts](https://www.terraform.io/d
 
 ### Credit Specification
 
-~> **NOTE:** Removing this configuration on existing instances will only stop managing it. It will not change the configuration back to the default for the instance type.
-
-Credit specification can be applied/modified to the EC2 Instance at any time.
-
 The `credit_specification` block supports the following:
 
 * `cpu_credits` - (Optional) Credit option for CPU usage. Valid values include `standard` or `unlimited`. T3 instances are launched as unlimited by default. T2 instances are launched as standard by default.