[mdspan.extents.cons] require conversions to index_type to be nonnegative

"nonnegative" is meaningless for a value of some arbitrary type which we've only required to be convertible to the integral type `index_type`, so this wording clearly intends to constrain the result of the conversion.

Author: CaseyCarter

source/containers.tex

@@ -19296,8 +19296,8 @@
 \item
 \tcode{sizeof...(exts) == 0} is \tcode{true}, or
 \item
-each element of \tcode{exts} is nonnegative and
-is representable as a value of type \tcode{index_type}.
+each element of \tcode{exts} is representable
+as a nonnegative value of type \tcode{index_type}.
 \end{itemize}
 \end{itemize}
 
@@ -19340,8 +19340,8 @@
 \item
 \tcode{N} is zero, or
 \item
-\tcode{exts[$r$]} is nonnegative and
-is representable as a value of type \tcode{index_type} for every rank index $r$.
+\tcode{exts[$r$]} is representable
+as a nonnegative value of type \tcode{index_type} for every rank index $r$.
 \end{itemize}
 \end{itemize}