KVM: s390: Clarify key argument for MEM_OP in api docs

Clarify that the key argument represents the access key, not the whole
storage key.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Link: https://lore.kernel.org/r/20220221143657.3712481-1-scgl@linux.ibm.com


Fixes: 5e35d0eb ("KVM: s390: Update api documentation for memop ioctl")
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>