message: add zip_iterator consistency check documentation
diff: @@ -118,6 +118,23 @@ public:
  * to the individual tuple elements get translated to the corresponding
  * iterator's references.
  *
  + * @note Two zip_iterators can only be compared if each individual pair of
  + * wrapped iterators has the same distance. Otherwise the behavior is
  + * undefined. This means that the only guaranteed safe way to use multiple
  + * zip_iterators is if they are all derived from the same iterator:
  + * ```
  + * Iterator i, j;
  + * auto it1 = make_zip_iterator(i, j);
  + * auto it2 = make_zip_iterator(i, j + 1);
  + * auto it3 = make_zip_iterator(i + 1, j + 1);
  + * auto it4 = it1 + 1;
  + * it1 == it2; // undefined
  + * it1 == it3; // well-defined false
  + * it3 == it4; // well-defined true
  + * ```
  + * This property is checked automatically in Debug builds and assumed in
  + * Release builds.
  + *
  * @see zip_iterator_reference
  * @tparam Iterators the iterators to zip together
  */
  