Add more doc

Author: nicolasstucki

library/src/scala/tasty/reflect/Core.scala

@@ -311,7 +311,10 @@ trait Core {
     /** Type tree representing a type bound written in the source */
     type TypeBoundsTree <: TypeOrBoundsTree
 
-    /** Type tree representing wildcard type bounds written in the source */
+    /** Type tree representing wildcard type bounds written in the source.
+     *  The wildcard type `_` (for example in in `List[_]`) will be a type tree that 
+     *  represents a type but has `TypeBound`a inside.
+     */
     type WildcardType <: TypeOrBoundsTree
 
   /** Type or bounds */