[ranges.syn] Add 'freestanding' markers per P1642R11

Author: jensmaurer

source/ranges.tex

@@ -225,12 +225,12 @@
   // \ref{range.as.rvalue}, as rvalue view
   template<@\libconcept{view}@ V>
     requires @\libconcept{input_range}@<V>
-  class as_rvalue_view;
+  class as_rvalue_view;                                         // freestanding
 
   template<class T>
     inline constexpr bool enable_borrowed_range<as_rvalue_view<T>> = enable_borrowed_range<T>;
 
-  namespace views { inline constexpr @\unspec@ as_rvalue = @\unspec@; }
+  namespace views { inline constexpr @\unspec@ as_rvalue = @\unspec@; }  // freestanding
 
   // \ref{range.filter}, filter view
   template<@\libconcept{input_range}@ V, @\libconcept{indirect_unary_predicate}@<iterator_t<V>> Pred>