[mdspan.layout.leftpad.cons] Add \expected.

The description of

    template<class LayoutLeftPaddedMapping>
      constexpr explicit(see below )
        mapping(const LayoutLeftPaddedMapping& other);

didn't separate Mandates and Preconditions. This commit adds an
\expects to separate the two.

Author: 1uc

source/containers.tex

@@ -23354,6 +23354,7 @@
 is \tcode{true}.
 
 \pnum
+\expects
 \begin{itemize}
 \item
 If \exposid{rank_} is greater than 1 and