P2630R4 submdspan

Editorial notes:
- [mdspan.submdspan.submdspan] Fix typo: "args" should say "slices".
- [mdspan.submdspan.overview] Change "subsection" to "subclause".
- [mdspan.submdspan.extents] Fix comma placement.
- [mdspan.submdspan.extents] "for each" instead of "for all".
- [mdspan.submdspan.submdspan, mdspan.syn] Fix cross-reference.
- [mdspan.submdspan.submdspan] Fix punctuation in itemized list.

Author: burblebee