Clarify join produces a string

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

@@ -152,7 +152,7 @@ The supported built-in functions are:
   * `index(list, elem)` - Finds the index of a given element in a list. Example:
       `index(aws_instance.foo.*.tags.Name, "foo-test")`
 
-  * `join(delim, list)` - Joins the list with the delimiter. A list is
+  * `join(delim, list)` - Joins the list with the delimiter for a resultant string. A list is
       only possible with splat variables from resources with a count
       greater than one. Example: `join(",", aws_instance.foo.*.id)`