Add documentation for compact()

website/source/docs/configuration/interpolation.html.md

@@ -80,6 +80,11 @@ The supported built-in functions are:
   * `base64encode(string)` - Returns a base64-encoded representation of the
     given string.
 
+  * `compact(list)` - Removes empty string elements from a list. This can be
+     useful in some cases, for example when passing joined lists as module
+     variables or when parsing module outputs.
+     Example: `compact(module.my_asg.load_balancer_names)`
+
   * `concat(list1, list2)` - Combines two or more lists into a single list.
      Example: `concat(aws_instance.db.*.tags.Name, aws_instance.web.*.tags.Name)`