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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Neato! `.. code-block: c++` should be: `.. code-block:: c++` -- did someone disable sphinx diagnostics or something? That should have been diagnosed...

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