docs: Document shuffle template func

Fixes #2393.

docs/content/templates/functions.md

@@ -97,6 +97,18 @@ One use case is the concatenation of elements in combination with `delimit`:
 ```
 
 
+### shuffle
+
+`shuffle` returns a random permutation of a given array or slice, e.g.
+
+```html
+{{ shuffle (seq 1 5) }}
+<!-- returns [2 5 3 1 4] -->
+
+{{ shuffle (slice "foo" "bar" "buzz") }}
+<!-- returns [buzz foo bar] -->
+```
+
 ### echoParam
 Prints a parameter if it is set.