Document this functionality

Author: therealcisse

docs/docs/reference/contextual/givens.md

@@ -105,6 +105,35 @@ instance named `ctx` is established by matching against the first half of the `p
 
 In each case, a pattern-bound given instance consists of `given` and a type `T`. The pattern matches exactly the same selectors as the type ascription pattern `_: T`.
 
+## Negated Givens
+
+Scala 2's somewhat puzzling behavior with respect to ambiguity has been exploited to implement the analogue of a "negated" search in implicit resolution, where a query Q1 fails if some other query Q2 succeeds and Q1 succeeds if Q2 fails. With the new cleaned up behavior these techniques no longer work. But there is now a new special type `scala.util.NotGiven` which implements negation directly.
+
+For any query type Q: NotGiven[Q] succeeds if and only if the implicit search for Q fails. Example:
+
+```scala
+
+import scala.util.NotGiven
+
+trait Tagged[A]
+
+case class Foo[A](value: Boolean)
+trait FooLowPrio {
+  implicit def fooDefault[A]: Foo[A] = Foo(true)
+}
+object Foo extends FooLowPrio {
+  implicit def fooNotTagged[A](implicit ev: NotGiven[Tagged[A]]): Foo[A] = Foo(false)
+}
+
+@main def main() =
+  implicit val taggedInt: Tagged[Int] = null
+
+  assert(implicitly[Foo[Int]].value) // fooDefault
+
+  assert(!implicitly[Foo[String]].value) // fooNotTagged
+
+```
+
 ## Given Instance Initialization
 
 A given instance without type or context parameters is initialized on-demand, the first