gh-135913: Document ob_refcnt, ob_type, ob_size (GH-135914)

* gh-135913: Document ob_refcnt, ob_type, ob_size

In `typeobj.rst`, instead of `:c:member:` it would be better to
use `.. c:member::` with a `:no-index:` option, see:

See ref. https://www.sphinx-doc.org/en/master/usage/domains/index.html#basic-markup

However, `c:member` currently does not support `:no-index:`.