[iterator.requirements.general] Clarify non-forward iterator

Author: frederick-vs-ja

source/iterators.tex

@@ -694,8 +694,9 @@
 do not specify complexity.
 
 \pnum
-Destruction of a non-forward iterator may invalidate pointers and references
-previously obtained from that iterator.
+Destruction of an iterator may invalidate pointers and references previously
+obtained from that iterator if its type does not meet the
+\oldconcept{ForwardIterator} requirements and does not model \libconcept{forward_iterator}.
 
 \pnum
 An \defnadj{invalid}{iterator}