gh-119180: Updates to PEP 649/749 docs

[JelleZijlstra]

• 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.

• Issue: Implement PEP 649 and PEP 749 #119180

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