KVM: s390: diagnose call documentation

Add some further documentation on the DIAGNOSE calls we support.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>

Documentation/virtual/kvm/hypercalls.txt

@@ -17,6 +17,9 @@ S390:
   S390 uses diagnose instruction as hypercall (0x500) along with hypercall
   number in R1.
 
+  For further information on the S390 diagnose call as supported by KVM,
+  refer to Documentation/virtual/kvm/s390-diag.txt.
+
  PowerPC:
   It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.
   Return value is placed in R3.