A tweak in PEP 689 -- Unstable C API tier #185

Hi,
When reviewing PEP 689 PR, we found a slightly better policy for moving an API from the unstable tier to the public tier. Instead of:

- The old name should remain available until the first incompatible change
- is made or the API is removed.
+ The old name should remain available until the API is deprecated or removed.

The policy of keeping old names means users don't need to make unnecessary changes when unstable API matures. But a deprecation means you'll need to stop using the API eventually.
And if anyone uses the unstable name, they've already signed up for removals without a deprecation period.

Is that OK?

The published docs have the new text, I'll revert if you have objections.
For the PEP there's a PR with a rendered preview.