Update the CPython PyObject_HasAttrString C API docs to mention that PyObject_HasAttr is a faster alternative if the same attribute name is going to be used multiple times.

We can model this after https://docs.python.org/3/c-api/call.html#c.PyObject_CallFunction which has the following note:

Note that if you only pass PyObject* args, PyObject_CallMethodObjArgs() is a faster alternative.