Add comments to generics example

tests/generics.pn

@@ -14,11 +14,15 @@ type int64 = object {
 # as possible. This syntax is very useful in cases like the built-in array type: it allows the syntax for it
 # to be just array[T, N], where T is the type of its elements and N is its size
 type Test<T: int64>[V: int64] = object {
+    ## This structure holds both the int64 type
+    ## and a value of type int64
     typeObj: T;
     value: V;
 }
 
 type Test2<T: Test>[V: Test] = object {
+    ## This structure holds both the Test
+    ## type and a (concrete) instance of Test
     typeObj: T;
     value: V;
 }