As shown by Play, users may be surprised by the semantic change to -Xsource:3, so that should be highlighted as a breaking change.

I've added a commit that strengthens the wording.

I would have used stronger language and stronger typeface.

Existing users **MUST** upgrade to -Xsource:3-cross to use future behavior.

-Xsource:3 or -Xsource:3-migration offers _only_ warnings.

**Don't delay!**

*Act* _NOW_!

@som-snytt I pushed a few more commits that give more of that flavor. as usual, we don't put lengthy explanations in release notes, so you might take a fresh look at the PR description at scala/scala#10573 and make sure it communicates what you think people need to know

I just added a paragraph to scala/scala#10573, the one that begins "Important:".

These notes are now at https://github.com/scala/scala/releases/tag/v2.13.13 . But they can still be modified; feel free to continue to use this PR to suggest improvements.