LWG4217 Clarify mdspan layout mapping requirements for rank == 0

Author: burblebee

source/containers.tex

@@ -21803,6 +21803,13 @@
 \pnum
 \returns
 $s_r$ as defined in \tcode{m.is_strided()} above.
+
+\pnum
+\begin{note}
+It is not required for \tcode{m.stride(r)} to be well-formed
+if \tcode{m.extents().rank()} is zero,
+even if \tcode{m.is_always_strided()} is \tcode{true}.
+\end{note}
 \end{itemdescr}
 
 \begin{itemdecl}