[libc++] [docs] [C++2b] Update status page with issues (and forgotten papers)...

[libc++] [docs] [C++2b] Update status page with issues (and forgotten papers) adopted in November 2020 and February 2021 virtual meetings.

Sources:
* https://github.com/cplusplus/draft/pull/4380 (November 2020)
* https://github.com/cplusplus/draft/pull/4523 (February 2021)
* https://github.com/cplusplus/draft/issues?q=is%3Aissue+LWG+Motion