LWG3102 Clarify span iterator and const_iterator behavior

Author: jensmaurer

source/containers.tex

@@ -9750,7 +9750,12 @@
 the program is ill-formed.
 
 \pnum
-The iterator type for span is a random access iterator and a contiguous iterator.
+The iterator types \tcode{span::iterator} and \tcode{span::const_iterator} are
+random access iterators\iref{random.access.iterators},
+contiguous iterators\iref{iterator.requirements.general}, and
+constexpr iterators\iref{iterator.requirements.general}.
+All requirements on container iterators\iref{container.requirements} apply to
+\tcode{span::iterator} and \tcode{span::const_iterator} as well. 
 
 \pnum
 All member functions of \tcode{span} have constant time complexity.