Fix Note text for NullValueRetriever

[SeanKilleen]

Resolves #2624.

The issue appears to be that I was using RST format when the file extension was MD. I was mistaken that RST can be used within the Markdown.

I'll resolve the symptom first and then I'll see if I can bring back the note box. If not, I'll merge as-is.

• [ ] Resolve symptom
• [ ] Bring back more styled note box

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] Performance improvement
• [ ] Refactoring (so no functional change)
• [x] Other (docs, build config, etc)

Checklist:

• [ ] I've added tests for my code. (most of the time mandatory)
• [ ] I have added an entry to the changelog. (mandatory)
• [ ] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.

[SeanKilleen]

OK, we've at least gotten rid of the regression.

I was unable to find reference for how to introduce the default formatting for info/warning boxes into ReadTheDocs + Sphinx + Markdown.

I'll probably have to leave it here. Feel free to merge and I think it'll at least be in a better place.

[SabotageAndi]

Thanks. This is enough for now.