Add info pointer for singleton and refinement types, scala#2933 (scala#2934)

* Add info pointer for singleton and refinement types, scala#2933

* Link to specific sections of the spec for literal and refined types

Author: vreuter

_overviews/scala3-book/types-others.md

@@ -21,7 +21,9 @@ Scala has several other advanced types that are not shown in this book, includin
 - Refinement types
 - Kind polymorphism
 
-For more details on these types, see the [Scala 3 Reference documentation][reference].
+For more details on most of these types, refer to the [Scala 3 Reference documentation][reference]. 
+For singleton types see the [literal types](https://scala-lang.org/files/archive/spec/3.4/03-types.html#literal-types) section of the Scala 3 spec, 
+and for refinement types, see the [refined types](https://scala-lang.org/files/archive/spec/3.4/03-types.html) section.