doc: document `!in` (#8546)

doc/docs.md

@@ -1046,15 +1046,18 @@ match mut x {
 ### In operator
 
 `in` allows to check whether an array or a map contains an element.
+To do the opposite, use `!in`.
 
 ```v
 nums := [1, 2, 3]
 println(1 in nums) // true
+println(4 !in nums) // true
 m := {
 	'one': 1
 	'two': 2
 }
 println('one' in m) // true
+println('three' !in m) // true
 ```
 
 It's also useful for writing boolean expressions that are clearer and more compact: