P0009R18 mdspan

Editorial changes:
 - Rephrase "may" in notes.
 - Give expression `sizeof...(OtherSizeTypes)` a name (`N`) to
   simplify the presentation.
 - Change "for all rank index `r`" to "for every rank index `r`".
 - Drop some extraneous parentheses.
 - Spell non-type template parameter `Args`, not `args`.

Author: jensmaurer