Improve documentation for typing.get_type_hints (#119928)

- Explicit list of what it does that is different from
  "just return __annotations__"
- Remove reference to PEP 563; adding the future import doesn't
  do anything to type aliases, and in general it will never make
  get_type_hints() less likely to fail.
- Remove example, as the Annotated docs already have a similar
  example, and it's unbalanced to have one example about this
  one edge case but not about other behaviors of the function.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>