website: Use a normal file-based template in example, and expand the explanation of $${thing}

website/docs/configuration-0-11/interpolation.html.md

@@ -451,14 +451,20 @@ Terraform 0.12 and later.
 Long strings can be managed using templates.
 [Templates](/docs/providers/template/index.html) are
 [data-sources](./data-sources.html) defined by a
-filename and some variables to use during interpolation. They have a
-computed `rendered` attribute containing the result.
+string with interpolation tokens (usually loaded from a file) and some variables
+to use during interpolation. They have a computed `rendered` attribute
+containing the result.
 
 A template data source looks like:
 
+```hcl
+# templates/greeting.tpl
+${hello} ${world}!
+```
+
 ```hcl
 data "template_file" "example" {
-  template = "$${hello} $${world}!"
+  template = "${file("templates/greeting.tpl")}"
   vars {
     hello = "goodnight"
     world = "moon"
@@ -472,7 +478,12 @@ output "rendered" {
 
 Then the rendered value would be `goodnight moon!`.
 
-Note that the double dollar signs (`$$`) are needed in inline templates. Otherwise Terraform will return an error.
+-> **Note:** If you specify the template as a literal string instead of loading
+a file, the inline template must use double dollar signs (like `$${hello}`) to
+prevent Terraform from interpolating values from the configuration into the
+string. This is because `template_file` creates its own instance of the
+interpolation system, with values provided by its nested `vars` block instead of
+by the surrounding scope of the configuration.
 
 You may use any of the built-in functions in your template. For more
 details on template usage, please see the