Improve documentation of CopyOrReference.

Other shall not doubt its right of existence in
times of perfect forwarding and convenient cv-modifiers.

dune/solvers/common/copyorreference.hh

@@ -21,6 +21,16 @@ namespace Solvers {
  *
  * If T is of reference type, the wrapper stores a reference
  * to the passed object. Otherwise it stores a copy.
+ *
+ * Internally, for reference type T, this wrapper stores a pointer, which
+ * results in CopyOrReference being copyable (etc.). This sets this wrapper
+ * apart from having a pure member type T in a class in particular.
+ * Furthermore, using this wrapper provides gclarity for the reader that type T
+ * may be both, of raw type or of reference type.
+ *
+ * Usage of this wrapper comes at the price of get()-semantics.
+ * As an alternative with pointer semantics consider using wrap_own_share but
+ * mind the dynamic overhead.
  */
 template<class T>
 class CopyOrReference