website: Index for `element` must be a non-negative integer (#26679)

* The index must be non-negative integer

and added instructions on how to get the last value in the list.

* Typo fix

Co-authored-by: Nick Fagerlund <nick@hashicorp.com>

website/docs/configuration/functions/element.html.md

@@ -19,7 +19,7 @@ element(list, index)
 ```
 
 The index is zero-based. This function produces an error if used with an
-empty list.
+empty list. The index must be a non-negative integer.
 
 Use the built-in index syntax `list[index]` in most cases. Use this function
 only for the special additional "wrap-around" behavior described below.
@@ -39,6 +39,15 @@ If the given index is greater than the length of the list then the index is
 a
 ```
 
+To get the last element from the list use [`length`](./length.html) to find
+the size of the list (minus 1 as the list is zero-based) and then pick the
+last element:
+
+```
+> element(["a", "b", "c"], length(["a", "b", "c"])-1)
+c
+```
+
 ## Related Functions
 
 * [`index`](./index.html) finds the index for a particular element value.