kvm: x86: Amend the KVM_GET_SUPPORTED_CPUID API documentation

Document the subtle nuances that KVM_CAP_X86_DISABLE_EXITS induces in
the KVM_GET_SUPPORTED_CPUID API.

Fixes: 4d5422ce ("KVM: X86: Provide a capability to disable MWAIT intercepts")
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>