bpo-2534: Restore PyObject_IsInstance() comment by vstinner · Pull Request #18345 · python/cpython

Restore PyObject_IsInstance() comment explaining why only tuples of
types are accepted, but not general sequence. Comment written by
Guido van Rossum in commit 03290ec
which implements isinstance(x, (A, B, ...)). The comment was lost in
a PyObject_IsInstance() optimization:
commit ec569b7.

Cleanup also the code. recursive_isinstance() is no longer recursive,
so rename it to object_isinstance(), whereas object_isinstance() is
recursive and so rename it to object_recursive_isinstance().