KVM: s390: add documentation for diag 501

Added documentation for diag 501, stating that no subfunctions are provided and
no parameters are used.

Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

Documentation/virtual/kvm/s390-diag.txt

@@ -78,3 +78,5 @@ DIAGNOSE function code 'X'501 - KVM breakpoint
 
 If the function code specifies 0x501, breakpoint functions may be performed.
 This function code is handled by userspace.
+
+This diagnose function code has no subfunctions and uses no parameters.