[clang] [clang][docs] Update CommandLineUsage and UsersManual with some flags… (PR #213342)

[Balázs Benics via cfe-commits]

steakhal wrote:

Could you change your branch such that we have a clean revert of the revert commit, then the fix you mentioned? This way I'd be able to see what I've already approved and if the fix looks correct to me.
Right now they are entangled. Use a force push for making this cleanup.

https://github.com/llvm/llvm-project/pull/213342