update docs

Signed-off-by: Pulak Kanti Bhowmick <[email protected]>

website/docs/cli/commands/terraform/terraform-shell.mdx

@@ -39,6 +39,10 @@ The command does the following:
 
 - Inside the shell, the user can execute all `terraform` commands using the native syntax
 
+- Sets the ATMOS_SHLVL environment variable to track shell nesting levels.
+    - If ATMOS_SHLVL is not set, it initializes to 1.
+    - If ATMOS_SHLVL is already set, it increments the value by 1 for each new shell.
+
 :::tip
 Run `atmos terraform shell --help` to see all the available options
 :::