Add known pitfalls to the article of nullable warnings and include 'Constructors` section in it.

[BartoszKlonowski]

This pull request closes #44184

It adds a note regarding the known pitfalls of nullable warning not always being issued and adds the example (well presented by the issue's author) to that "Known pitfalls" section as a "Constructors" case.

---

Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/nullable-warnings.md	Resolve nullable warnings
docs/csharp/nullable-references.md	Nullable reference types

[BillWagner]

Thanks again @BartoszKlonowski

I had a few comments, and then this is ready.

[BillWagner]

I think the build error has been addressed. Closing & Reopening to check.

[BartoszKlonowski]

I think the build error has been addressed. Closing & Reopening to check.

I saw that there are two files that are included in the check and that have warnings in them. artificial-intelligence.md and ai-extensions.md. There's a note that it may happen at times that other files will be included, but I can't access the linked instruction on how to deal with such scenario, so I will count on your help, @BillWagner 🙏