[libc++][doc] Update formatting status.

Reduced the details of the non-chrono formatting information. This has
been shipped and these details part of P0645 which is still documented.
Removing this information keeps the information up-to-date.

Adds the formatters required for the types chrono namespace.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D122735