Restore PyObject_IsInstance() comment (GH-18345)

Restore PyObject_IsInstance() comment explaining why only tuples of
types are accepted, but not general sequence. Comment written by
Guido van Rossum in commit 03290ecbf1661c0192e6abdbe00ae163af461d77
which implements isinstance(x, (A, B, ...)). The comment was lost in
a PyObject_IsInstance() optimization:
commit ec569b794737be248671d0dfac11b664fc930eef.

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().