[Nullable Reference Types] Provide informative notes and examples on diagnostics

[BillWagner]

We likely need some informative examples for static null state flow analysis and the diagnostics that might be produced.

As we complete the normative information, we will have a better idea what examples or notes will help readers understand the feature, its benefits, and the diagnostics.

[BillWagner]

Let's add this to the end of the agenda (after discussing NRT, #1092, #1093). If we have some guidance on how far to go on this route, it would help me create a PR for November.