Add an FAQ entry and add a few lines to another one. Document the use of CCC_OVERRIDE_OPTIONS for injecting GCC toolchain path.
Thanks for improving the documentation. Some comments, the documentation should be generic over specific cmake invocations / gcc versions and errors.
I'd just say something like "If building OpenMP with LLVM_ENABLE_RUNTIMES, OpenMP should not be enabled as a project in LLVM_ENABLE_PROJECTS because it is enabled by default."
No need to be specific to GCC 9.1.0.
Don't give a specific cmake invocation, we can just refer to the existing documentation for setting up CMake as above.