KVM: Add documentation for KVM_CAP_NR_VCPUS

Document KVM_CAP_NR_VCPUS that can be used by the userspace to determine
maximum number of VCPUs it can create with the KVM_CREATE_VCPU ioctl.

Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Avi Kivity <avi@redhat.com>