[INF-0002] Fill out sections on Validations in Sema and Codegen

[damyanp]

proposals/infra/INF-0002-root-signature-in-clang.md

These sections need to be filled out with the relevant information:

• Validations in Sema
• Validations during Codegen

Also check the rest of the document is up to date and consistent with the plan for validations and diagnostics.