[libc++][doc] Updates format status.

The paper
- P2286R8 Formatting ranges
is fully implemented modulo its feature test macro. This macro has been
revised by
- LWG3750 Too many papers bump __cpp_lib_format
The new macro depends on
- P2585R0 Improving default container formatting
This paper revises parts of P2286R8 and adds new formatter
specializations. The specialization for debug strings has some wording
issues, which is addresses in this paper
- P2733R0 Fix handling of empty specifiers in std::format

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D142153