doc: document fixed size init syntax

doc/docs.md

@@ -813,6 +813,8 @@ fnums[2] = 100
 println(fnums) // => [1, 10, 100]
 println(typeof(fnums).name) // => [3]int
 
+fnums2 := [1, 10, 100]! // short init syntax that does the same (the syntax will probably change)
+
 anums := fnums[0..fnums.len]
 println(anums) // => [1, 10, 100]
 println(typeof(anums).name) // => []int