Clarify stability of Stable ABI

[encukou]

I used some unfortunate wording in the docs:

The extension will work without recompilation with all Python 3 releases from the specified one onward [...]

Clarify that compiling for the Stable ABI will prevent ABI issues (missing symbols or data corruption due to changed layouts/signatures). But, behaviour can still change per PEP-387 (though we should keep it stable if we can).

Linked PRs

• gh-127253: Note that Stable ABI is about ABI stability #127254
• [3.13] gh-127253: Note that Stable ABI is about ABI stability (GH-127254) #127557
• [3.12] gh-127253: Note that Stable ABI is about ABI stability (GH-127254) #127558