Fix test case and documentation.

Author: lidaisy

docs/_docs/reference/experimental/valhalla.md

@@ -33,7 +33,7 @@ Valhalla traits can extend `Any` or universal traits.
 ```scala
 import scala.annotation.valhalla
 
-@valhalla trait(val x: Int, val y: Int) ValhallaTrait extends Any
+@valhalla trait ValhallaTrait(val x: Int, val y: Int) extends Any
 
 ```
 

library/src/scala/annotation/valhalla.scala

@@ -10,4 +10,5 @@ import scala.language.`2.13`
  * class as specified in https://openjdk.org/jeps/401.
  */
 
+@scala.annotation.experimental
 final class valhalla extends StaticAnnotation

tests/valhalla/pos/ValhallaExtendsNonValhallaAnyTrait.scala

@@ -1,19 +1,14 @@
 import scala.annotation.valhalla
 
-/**
- * Valhalla classes and traits cannot extend non-Valhalla classes or traits
- */
-
 trait AnyTrait extends Any:
   val a: Int = 2
   def add(x:Int, y:Int): Int
 
-
 @valhalla
-trait TraitExtendsAnyTrait extends Any with AnyTrait: // error
+trait TraitExtendsAnyTrait extends Any with AnyTrait:
   def addOne(x: Int): Int
 
 @valhalla
-class VVC extends AnyVal with AnyTrait: // error
+class VVC extends AnyVal with AnyTrait:
   def add(x:Int, y:Int): Int = x + y
   def addOne(x: Int): Int = x + 1