Automate Nullable Annotation Change Document

[jeffhandley]

dotnet/docs#21202 has been created by hand thus far; we could automate it by scanning the differences in the ref assemblies. The tool could use the already-documented changes as the baseline and we could run the tool for every Preview/RC/GA release to update the document.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[jeffhandley]

With the low number of changes we've made to annotations in 6.0, and the effectiveness of manually documenting them, we won't pursue automating this document generation.