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

gh-141004: Document old provisional names for vectorcall & code API

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.