Rework `PyObject_IsSubclass` documentation 

`PyObject_IsSubclass` [docs](https://docs.python.org/3/c-api/object.html#c.PyObject_IsSubclass) should probably be worded more succinctly and to the point

_Originally posted by @erlend-aasland in https://github.com/python/cpython/pull/102554#pullrequestreview-3238838324_
            


### Linked PRs
* gh-140236
* gh-140346