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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@AaronBallman After moving the docs, none of the code blocks are rendering:
https://clang.llvm.org/docs/InternalsManual.html#verifying-diagnostics

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