Document the precise behavior of the constructor taking an initializer_list

dune/common/diagonalmatrix.hh

@@ -108,6 +108,14 @@ namespace Dune {
       : diag_(diag)
     {}
 
+    /** \brief Construct diagonal matrix from an initializer list
+     *
+     * The elements of the list are copied into the diagonal elements of the matrix.
+     * If the initializer list is shorter than the matrix diagonal (which has n elements),
+     * the remaining matrix diagonal elements are left uninitialized.  If the initializer
+     * list is longer, than only the first n elements will be copied into the matrix
+     * diagonal.
+     */
     DiagonalMatrix (std::initializer_list<K> const &l)
     {
       std::copy_n(l.begin(), std::min(static_cast<std::size_t>(rows),