[mdspan.mdspan.overview] Add default template arguments for mdspan (#6018)

We usually restate the default template arguments in the class synopsis,
so that the information is in one place and one does not need to also refer
to the header synopsis.

Author: hewillk

source/containers.tex

@@ -20149,7 +20149,8 @@
 
 \begin{codeblock}
 namespace std {
-  template<class ElementType, class Extents, class LayoutPolicy, class AccessorPolicy>
+  template<class ElementType, class Extents, class LayoutPolicy = layout_right,
+           class AccessorPolicy = default_accessor<ElementType>>
   class mdspan {
   public:
     using extents_type = Extents;