Free-threading documentation should mention PyList_GET_ITEM

[crusaderky]

https://docs.python.org/3/howto/free-threading-extensions.html#borrowed-references mentions that PyList_GetItem is thread-unsafe due to borrowed references. PyList_GET_ITEM should be added next to it.

Linked PRs

• gh-137043: DOC: Free-threading howto should mention PyList_GET_ITEM #137042
• [3.14] gh-137043: mention PyList_GET_ITEM as unsafe borrowed API in free-threading docs (GH-137042) #137045
• [3.13] gh-137043: mention PyList_GET_ITEM as unsafe borrowed API in free-threading docs (GH-137042) #137046