Update documentation of release tracking analyzer

Hi there,

the implementation of [`ReleaseTrackingHelper`](https://github.com/dotnet/roslyn-analyzers/blob/main/src/Tools/GenerateDocumentationAndConfigFiles/ReleaseTrackingHelper.cs) seems to differ from [Release Tracking Analyzer documentation](https://github.com/dotnet/roslyn/blob/main/src/RoslynAnalyzers/Microsoft.CodeAnalysis.Analyzers/ReleaseTrackingAnalyzers.Help.md). Especially the `### Changed Rules` parsing is changed and not reflected in the documentation, which means `AnalyzerReleases.Shipped.md` is now producing warnings like

> AnalyzerReleases.Shipped.md(13,1,13,38): warning RS2007: Analyzer release file 'AnalyzerReleases.Shipped.md' has a missing or invalid release header 'Rule ID | Category | Severity | Notes'.

It would be nice to have an updated documentation, otherwise one need to fetch the details from the sources.