Add clarification for abstract function bodies.

I find it clearer to explicitly state that `abstract int foo() { return 0; }` renders the encompassing class un-instantiatable, even though all function bodies are available.

spec/attribute.dd

@@ -698,7 +698,9 @@ $(P
 $(P
         Functions declared as abstract can still have function
         bodies. This is so that even though they must be overridden,
-        they can still provide $(SINGLEQUOTE base class functionality.)
+        they can still provide $(SINGLEQUOTE base class functionality),
+        e.g. through $(D super.foo()) in a derived class.
+        Note that the class is still abstract and cannot be instantiated directly.
 )
 
 $(SECTION3 $(LNAME2 uda, User Defined Attributes),