Show synthesized extension instance name in docs

Author: odersky

docs/docs/reference/contextual/extension-methods.md

@@ -156,7 +156,7 @@ given listOps: AnyRef {
   def [T](xs: List[T]) second = xs.tail.head
   def [T](xs: List[T]) third: T = xs.tail.tail.head
 }
-given AnyRef {
+given given_largest_of_List_T: AnyRef {
   def [T](xs: List[T]) largest (given Ordering[T])(n: Int) =
     xs.sort.takeRight(n)
 }