Document constants like `Py_tp_repr`

[encukou]

We don't document type slots like Py_tp_repr individually, so they don't show up in search or for Intersphinx.

Constants like these also don't have notes like “Part of Stable ABI since version 3.5”. (The script dismisses them with the comment “all undocumented”.)

Linked PRs

• gh-138189: Document type slots, and other constants, as part of Limited API #138190
• [3.14] gh-138189: Document type slots, and other constants, as part of Limited API (GH-138190) #141131
• [3.13] gh-138189: Document type slots, and other constants, as part of Limited API (GH-138190) #141193
• gh-138189: Link references to type slots #141410
• [3.14] gh-138189: Link references to type slots (GH-141410) #141718