Document all flags

The flag -freduction-depth appears nowhere in the manual. I found its details in DynFlags, so I then looked up this flags neighbors -fconstraint-solver-iterations and -fcontext-stack. I didn't find those in the manual either. We should check to make sure that each user-facing flag is documented. At least the first one is certainly user-facing, as we suggest twiddling it in an error message.