[3.14] gh-141004: Document old provisional names for vectorcall/code API (GH-143568) (GH-143828)

For vectorcall API, the underscore-prefixed provisional names
continue to be available per PEP 590:
https://peps.python.org/pep-0590/#finalizing-the-api
Use `versionadded`, `versionchanged`, and `:no-typesetting:`
to mark this up.

For PyCode API, use `:no-typesetting:` rather than `index::` so that
these are semantically documented as C functions.
(cherry picked from commit 7e8a1b5061313e03f99bc2ffa2062702bcc76426)

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