Also - improve xml docs.

This PR reverts that change and a similar diagnostic will be implemented as an analyzer with https://github.com/dotnet/roslyn-analyzers/issues/1727.

Fixes #25748

Fixes #27627

This change also includes:
1. Added resource strings and unit tests for argument exceptions for CFG APIs.
2. Move flow specific operations into FlowAnalysis namespace.

See #26339 for the primary RCA.

This change corrects an additional edge case for ITextBuffer instances created before VisualStudioWorkspace attaches event handlers to ITextBufferFactoryService and IProjectionBufferFactoryService. This case is only believed to occur in IDE initialization scenarios where the workspace load is triggered by opening a document.

Fixes #24310