Document expectations around when rebootstrapping is needed for pre-releases

[MichaelSimons]

We should do a better job at documenting whether preview/RC releases require rebootstrapping.

[MichaelSimons]

This is documented at https://github.com/dotnet/source-build/blob/main/Documentation/VMR-re-bootstrapping.md#when-is-it-appropriate-to-re-bootstrap. That being said I think we should call out rebootstrapping is required in the release announcement for previews/when a re-bootstrap is required.

[NikolaMilosavljevic]

@MichaelSimons is the expectation here for the release pipeline to automatically detect if during current release (Preview/RC) there was a rebootstrap and new toolset was picked up?