[libc++][docs] Confirm that P2372R3 has been implemented (#148573)

In #125921, the changes requested by P2372R3 were completed and tested
together with corresponding `chrono` types. But that PR didn't mention
P2372R3. The `__cpp_lib_format` FTM was even bumped by an earlier PR
#98275.

This PR confirms that P2372R3 was completed in LLVM 21 (together with P1361R2).
Closes #100043

GitOrigin-RevId: 4177bfdb466855fed5e498774ac517b5cad2a995
2 files changed