Document unstable linker flags in the unstable book This PR: - removes the unstable linker flavors from the stable documentation - documents the unstable `-C linker-flavor` values in the unstable book (including the ones previously described as stable) - documents the unstable `-C link-self-contained` components in the unstable book I don't know if these have some common reviewers, but if not r? `@petrochenkov` as the most knowledgeable person in this area. Please feel free to reassign, I know that you are quite busy.