KVM: x86: hyper-v: Mention SynDBG CPUID leaves in api.rst

We forgot to update KVM_GET_SUPPORTED_HV_CPUID's documentation in api.rst
when SynDBG leaves were added.

While on it, fix 'KVM_GET_SUPPORTED_CPUID' copy-paste error.

Fixes: f97f5a56

 ("x86/kvm/hyper-v: Add support for synthetic debugger interface")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200924145757.1035782-2-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>