gh-119180: Updates to PEP 649/749 docs (#134640)

- Mention (again) that `type.__annotations__` is unsafe. It is now safe
  when using only classes defined under PEP 649 semantics, but not with
  classes defined using `from __future__ import annotations`.
- Mention that annotations on instances no longer work. There was already
  an issue about this.
- Mention the general changes in the "Porting to Python 3.14" section.
- `annotationlib` was proposed by PEP-749, not PEP-649.

Co-authored-by: Emma Smith <emma@emmatyping.dev>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>