Adding documentation for TF_CLI_CONFIG_FILE environment variable (#20834)

Fixes #15849

main.go

@@ -231,7 +231,12 @@ func wrappedMain() int {
 
 func cliConfigFile() (string, error) {
 	mustExist := true
-	configFilePath := os.Getenv("TERRAFORM_CONFIG")
+
+	configFilePath := os.Getenv("TF_CLI_CONFIG_FILE")
+	if configFilePath == "" {
+		configFilePath = os.Getenv("TERRAFORM_CONFIG")
+	}
+
 	if configFilePath == "" {
 		var err error
 		configFilePath, err = ConfigFile()

website/docs/commands/cli-config.html.markdown

@@ -32,6 +32,9 @@ CLI configuration file, even though Windows Explorer may _display_ its name
 as just `terraform.rc`. Use `dir` from PowerShell or Command Prompt to
 confirm the filename.
 
+The location of the Terraform CLI configuration file can also be specified
+using the `TF_CLI_CONFIG_FILE` [environment variable](/docs/commands/environment-variables.html).
+
 ## Configuration File Syntax
 
 The configuration file uses the same _HCL_ syntax as `.tf` files, but with

website/docs/commands/environment-variables.html.md

@@ -113,3 +113,11 @@ This is a purely cosmetic change to Terraform's human-readable output, and the
 exact output differences can change between minor Terraform versions.
 
 For more details see [Running Terraform in Automation](https://learn.hashicorp.com/terraform/development/running-terraform-in-automation).
+
+## TF_CLI_CONFIG_FILE
+
+The location of the [Terraform CLI configuration file](/docs/commands/cli-config.html).
+
+```shell
+export TF_CLI_CONFIG_FILE="$HOME/.terraformrc-custom"
+```