RFR: Show null restriction markers in javadoc.

[Jan Lahoda]

Adds null markers inside generated javadoc like this:
<img width="1144" height="528" alt="null-markers-javadoc" src="https://github.com/user-attachments/assets/b97736ff-f240-4c88-b874-32a8f7c8743c" />

Not sure if this is the ideal UI, and this is the best way to do implement it.

I didn't add tests yet, given the representation may change with experience.

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Commit messages:
 - Show null restriction markers in javadoc.

Changes: https://git.openjdk.org/valhalla/pull/2022/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2022&range=00
  Stats: 17 lines in 2 files changed: 17 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/2022.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2022/head:pull/2022

PR: https://git.openjdk.org/valhalla/pull/2022