[BTree][NFC] Rephrase some comments

[lorentey]

Change some unfortunate wording around invariant violations.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[vanvoorden]

@lorentey I believe soundness would also be a legit option to replace those.

FWIW… there might also be examples of outdated language that can't be deleted easily like comments… there could be constant strings passed to logger or even symbols like function names that contain the outdated language. You might also for this specific diff think about leaving these comments in but just migrating the language to something modern and inclusive as a guide for future engineers to migrate future symbols.

cc @Serena748

[lorentey]

Yep.

[lorentey]

C.f. #426

[lorentey]

I'm not particularly fond of the idea that assertions need to explicitly explain that they're for checking invariants. (That is what assertions are for.) Therefore, the right fix was to remove these.

Please file PRs or issues if you find any similar cases. These were found by an automated tool; I find it encouraging that the only hits were in experimental code that hasn't yet gone through the usual implementation/style review.