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

[Vlad Serebrennikov via cfe-commits]

Endilll 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_asserts etc.). I'm sure you are aware that libc++ uses -verify too.

While I'm sympathetic to the intent, making this more public bring up the question what are we going to promise the users. I believe status quo is that we can do whatever we feel like to, coordinating it with libc++ people, which sit at a proverbial next desk to us. So I'm not opposed to this, but I'd like to see how we going to address this concern.

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