[clang] Move documentation about -verify from a header to public docs (PR #73694)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

Would it make sense to document this more publicly? While it's designed to be an internal tool, it's really useful for people who want to make sure their library produces high quality diagnostics (e.g. `nodiscard`, `static_assert`s etc.). I'm sure you are aware that libc++ uses `-verify` too.

https://github.com/llvm/llvm-project/pull/73694