KVM: Document KVM_SET_SIGNAL_MASK as universal

KVM_SET_SIGNAL_MASK is implemented in generic code and isn't x86
specific, so document it as being applicable for all architectures.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: kvm@vger.kernel.org
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>