docs: kvm: Convert s390-diag.txt to ReST format

This file is almost in ReST format. Just one change was
needed:

    - Add markups for a literal block and change its indentation.

While here, use the standard markup for the document title.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/index.rst

@@ -17,6 +17,7 @@ KVM
    msr
    nested-vmx
    ppc-pv
+   s390-diag
    vcpu-requests
 
    arm/index

Documentation/virt/kvm/s390-diag.txt → Documentation/virt/kvm/s390-diag.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
 The s390 DIAGNOSE call on KVM
 =============================
 
@@ -16,12 +19,12 @@ DIAGNOSE calls by the guest cause a mandatory intercept. This implies
 all supported DIAGNOSE calls need to be handled by either KVM or its
 userspace.
 
-All DIAGNOSE calls supported by KVM use the RS-a format:
+All DIAGNOSE calls supported by KVM use the RS-a format::
 
---------------------------------------
-|  '83'  | R1 | R3 | B2 |     D2     |
---------------------------------------
-0        8    12   16   20           31
+  --------------------------------------
+  |  '83'  | R1 | R3 | B2 |     D2     |
+  --------------------------------------
+  0        8    12   16   20           31
 
 The second-operand address (obtained by the base/displacement calculation)
 is not used to address data. Instead, bits 48-63 of this address specify