docs: add example of using array decomposition (#7720)

doc/docs.md

@@ -210,6 +210,12 @@ fn sum(a ...int) int {
 println(sum())    // Output: 0
 println(sum(1))   //         1
 println(sum(2,3)) //         5
+
+// using array decomposition
+a := [2,3,4]
+println(sum(a...))  // <-- using postfix ... here. output: 9
+b := [5, 6, 7]
+println(sum(b...)) // output: 18
 ```
 
 ## Symbol visibility