message: Add more details about TreeNode requirements.
diff: // TreeNode concept:
  //
  // - `Container<T>& Children()`:
  -// Returns reference to a STL-like container containing the node's children.
  +// Returns reference to a STL-like container or a range containing the node's
  +// children.
  +// The Container must at least support `begin()` and `end()` methods.
  +// Pointer and iterator stability is not required.
  //
  // To check whether class `T` fulfills the concept one can use the
  // `TreeNodeTraits<T>` traits struct. It is defined only for classes fulfilling
  