[libc++][doc] Update format paper status.

- Fixes paper number P1862 -> P1868. (The title was correct.)
- Marks P1868 as in progress.
- Marks P1892 as in progress.
- Marks LWG-3327 as nothing to do, since the wording change doesn't
  impact the code. (Also updated on the general C++20 status page.)