[span.iterators] Specify iterator value_type and reference (#4062)

Author: JohelEGP

source/containers.tex

@@ -11236,7 +11236,11 @@
 requirements\iref{random.access.iterators},
 and
 meets the requirements for
-constexpr iterators\iref{iterator.requirements.general}.
+constexpr iterators\iref{iterator.requirements.general},
+whose value type is \tcode{value_type} and
+whose reference type is \tcode{reference}.
+
+\pnum
 All requirements on container iterators\iref{container.requirements} apply to
 \tcode{span::iterator} as well.
 \end{itemdescr}