Scaladoc tool: render @deprecated correctly even when named arguments weren't used

[HarrisL2]

Fixes #20118

Still need a test case for this issue using jsoup

[SethTisue]

I'm busy at the Lightbend company meeting the whole rest of the week, then I have some vacation days, so the earliest I could return to it is around Nov 21. @HarrisL2 feel free to run with it if you like. if you end up not having time either, I'll come back to it eventually

[HarrisL2]

I'm not able to write a test case for this change without making really specific checks. Ideally it would be a general test framework like SignatureTest.scala, where we can check the generated attributes of each member with an expected result. I don't think this framework exists at the moment.

There are test cases about annotations, but those are for annotations that affect the generated signature, whereas the @deprecated annotation is a special case that is added to the attribute section, which is not tested for.

I have added test cases to scaladoc-testcases/src/tests/deprecated.scala, but it should be noted that it currently does not check the behavior of annotations. It only checks signatures through Deprecated in TranslatableSignaturesTestCases.scala.

[SethTisue]

I hope this PR will be considered mergeable even though our ambition level stopped short of proper automated testing for it. A JSoup-based test would be annoying fragile, I think.

I've rebased it and marked it ready for review.

@KacperFKorban I'm not sure if you're currently available for reviewing this sort of PR; let me know if you think we should try to find a different reviewer.

[KacperFKorban]

Thanks for the fix, it looks good overall!
I wouldn't stress about adding tests for annotations, you would have to implement the whole infrastructure for it as well 😅
I added a small style related suggestion.