Clarify use of backend-config file (#35389)

website/docs/language/backend/index.mdx

@@ -183,6 +183,16 @@ naming pattern. Terraform will not prevent you from using other names but follow
 this convention will help your editor understand the content and likely provide
 better editing experience as a result.
 
+You can use the `-backend-config` argument on the CLI to provide a partial backend configuration. You can either specify a file containing a partial configuration or specify a set of key-value pairs. The configuration must already include a `backend` block to use this argument. Terraform applies the configurations to the arguments in the `backend` block, but it does not change the backend type. In the following example, Terraform stores state in the default backend using the arguments supplied in the `-backend-config` argument:
+
+```hcl
+terraform {
+  backend {
+    # values provided by backend-config file
+  }
+}
+```
+
 ### Command-line key/value pairs
 
 The same settings can alternatively be specified on the command line as