[doxygen] Add documentation to constexpr.hh.

dune/common/std/constexpr.hh

 #ifndef DUNE_COMMON_STD_CONSTEXPR_HH
 #define DUNE_COMMON_STD_CONSTEXPR_HH
 
-#if HAVE_CONSTEXPR
+/**
+ * \file
+ * \brief Definition of the DUNE_CONSTEXPR macro
+ */
+
+#if HAVE_CONSTEXPR || defined(DOXYGEN)
+
+/**
+ * \brief Set method or expression constexpr if supported by the compiler.
+ * \code
+   #include <dune/common/std/constexpr.hh>
+ * \endcode
+ *
+ * This is a preprocessor define which can be used to mark expressions or
+ * functions as constexpr. C++11 introduced constexpr, C++1y will extend
+ * constexpr's use cases.
+ */
 #define DUNE_CONSTEXPR constexpr
+
 #else // #if HAVE_CONSTEXPR
 #define DUNE_CONSTEXPR
 #endif // #else // #if HAVE_CONSTEXPR