Update compatibility docs

docs/user-guide/compatibility.md

@@ -4,6 +4,8 @@ TFLint interprets the [Terraform language](https://developer.hashicorp.com/terra
 
 The parser supports Terraform v1.x syntax and semantics. The language compatibility on Terraform v1.x is defined by [Compatibility Promises](https://developer.hashicorp.com/terraform/language/v1-compatibility-promises). TFLint follows this promise. New features are only supported in newer TFLint versions, and bug and experimental features compatibility are not guaranteed.
 
+The latest supported version is Terraform v1.8.
+
 ## Input Variables
 
 Like Terraform, TFLint supports the `--var`,` --var-file` options, environment variables (`TF_VAR_*`), and automatically loading variable definitions (`terraform.tfvars` and `*.auto.tfvars`) files. See [Input Variables](https://developer.hashicorp.com/terraform/language/values/variables).
@@ -117,9 +119,11 @@ The values below are state-dependent and cannot be determined statically, so TFL
 - `data.<DATA TYPE>.<NAME>`
 - `self`
 
-## Built-in Functions
+## Functions
+
+[Built-in Functions](https://developer.hashicorp.com/terraform/language/functions) are fully supported. However, functions such as [`plantimestamp`](https://developer.hashicorp.com/terraform/language/functions/plantimestamp) whose return value cannot be determined statically will return an unknown value.
 
-[Built-in Functions](https://developer.hashicorp.com/terraform/language/functions) are fully supported.
+[Provider-defined functions](https://www.hashicorp.com/blog/terraform-1-8-adds-provider-functions-for-aws-google-cloud-and-kubernetes) always return unknown values, except for `provider::terraform::*` functions.
 
 ## Dynamic Blocks