gh-110803: Reorganize docs for what PyType_Slot doesn't cover (GH-110813)

* gh-110803: Reorganize docs for what PyType_Slot doesn't cover

- Cover the offset fields first
- Mention the old alternative for MANAGED flags, which is needed
  to support older Pythons
- De-emphasize the internal flags: use an inline list.
- Add a note to PyMemberDef saying what to do with it

* Remove an older draft...