[iterator.synopsis] Add specialization specified in [iterator.traits]/3 (#961)

Author: JohelEGP

source/iterators.tex

@@ -686,6 +686,7 @@
   // \ref{iterator.primitives}, primitives:
   template<class Iterator> struct iterator_traits;
   template<class T> struct iterator_traits<T*>;
+  template<class T> struct iterator_traits<const T*>;
 
   struct input_iterator_tag { };
   struct output_iterator_tag { };