[3.14] gh-141004: Mark up constants for `PyOS_double_to_string` (GH-143867) (GH-144146)

This ensures they show up as C macros in search and the Sphinx inventory.
(cherry picked from commit fb690c38cafb15e4ccddc52aba37f9470ce201a0)

Co-authored-by: Petr Viktorin <encukou@gmail.com>