[2218] (trivial) documentation for DomainTreeNodeChain copy ctor.

src/lib/datasrc/memory/domaintree.h

@@ -697,6 +697,9 @@ public:
                                          isc::dns::NameComparisonResult::EQUAL)
     {}
 
+    /// \brief Copy constructor.
+    ///
+    /// \exception None
     DomainTreeNodeChain(const DomainTreeNodeChain<T>& other) :
         level_count_(other.level_count_),
 	last_compared_(other.last_compared_),