gh-148961 : Use parsed-literal for the slot column legend in c-api/typeobj.rst

[Chang-LeHung]

The two small legends under the PyTypeObject "Quick Reference" table in
Doc/c-api/typeobj.rst explain what each character in the "D" (default)
and "I" (inheritance) columns means. They were wrapped in
.. code-block:: none, so reST inline markup inside them was rendered
verbatim:

• *PyType_Ready* and *NULL* showed literal asterisks.
• PyType_Ready was plain text instead of a cross-reference link.
• NULL was plain text instead of the NULL inline literal used
  throughout the rest of the page.

Before:

After:

• Issue: Docs: inline markup in c-api/typeobj.rst legend is not rendered because of .. code-block:: none #148961

---

📚 Documentation preview 📚: https://cpython-previews--148962.org.readthedocs.build/

[StanFromIreland]

These break translations, see #135828.

[Chang-LeHung]

Thanks for the pointer to #135828.

If we want to drop parsed-literal from this PR, I see two reasonable replacements:

Option A — keep code-block:: none, just remove the stray emphasis markers

In the original second block, *PyType_Ready* and *NULL* are wrapped in asterisks, but code-block:: renders them verbatim, so they show up as literal * in the output. The first block doesn't have them. The minimal fix is just to remove those asterisks so the second block is consistent with the first:

   .. code-block:: none

-     X - type slot is inherited via *PyType_Ready* if defined with a *NULL* value
+     X - type slot is inherited via PyType_Ready if defined with a NULL value
      % - the slots of the sub-struct are inherited individually
      G - inherited, but only in combination with other slots; see the slot's description
      ? - it's complicated; see the slot's description

The visual layout stays exactly as it is today; we give up the cross-reference / inline-literal improvements.

Option B — switch to a line block (|)

A plain reST line block: no parsed-literal, no syntax-highlighting code path, translation-safe, and inline :c:func: / NULL still work. Visually it's a compact multi-line paragraph (no bullets, no extra spacing) — close to the original block-style legend, just without the gray code-block background:

   **"D"**:  default (if slot is set to ``NULL``)

   | ``X`` -- :c:func:`PyType_Ready` sets this value if it is ``NULL``
   | ``~`` -- :c:func:`PyType_Ready` always sets this value (it should be ``NULL``)
   | ``?`` -- :c:func:`PyType_Ready` may set this value depending on other slots

   Also see the inheritance column ("I").

   **"I"**:  inheritance

   | ``X`` -- type slot is inherited via :c:func:`PyType_Ready` if defined with a ``NULL`` value
   | ``%`` -- the slots of the sub-struct are inherited individually
   | ``G`` -- inherited, but only in combination with other slots; see the slot's description
   | ``?`` -- it's complicated; see the slot's description

make html with --fail-on-warning passes locally on both options.

A keeps the diff minimal; B preserves the original motivation of the PR (PyType_Ready as a cross-reference, NULL as an inline literal).