[Clang][Docs] Add new offloading flags to the clang documentation

Summary:
Some previous patches introduced the `--offload-new-driver` flag, which
is a generic way to enable the new driver, and the `--offload-host-only`
and `--offload-device-only` flags which allow users to compile for one
side, making it easier to inspect intermediate code for offloading
compilations. This patch just documents them in the command line
reference.