Synchronize improvements to cppreference once it becomes editable again

[frederick-vs-ja]

It's probable that some improvements will be made when porting contents from cppreference. Currently (2025-12-12), cppreference is not editable. We should record these improvements somewhere and synchronize them to cppreference whenever possible.

[frederick-vs-ja]

• https://en.cppreference.com/w/cpp/preprocessor.html
  • Perhaps we should rephrase "non-standard extension" to "extension" because it's unlikely to categorize some extension as standard.