[libc++][doc] Adds more issue status labels.

A followup to D111458 adding more labels to LWG-issues. This should add
the labels for the not completed chrono, format, ranges, and spaceship
issues.

Some minor formatting cleanups along the way.

Reviewed By: #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D111935