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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

It would also be nice to fix the link to full `-verify` docs in https://clang.llvm.org/docs/InternalsManual.html#the-diagnosticconsumer-interface

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