Documentation: Add MDS vulnerability documentation

Add the initial MDS vulnerability documentation.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130

(cherry picked from commit 62d4d464fd5f465de4afc55e267d994f6e103d37)
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Documentation/admin-guide/hw-vuln/index.rst

@@ -10,3 +10,4 @@ are configurable at compile, boot or run time.
    :maxdepth: 1
 
    l1tf
+   mds

Documentation/admin-guide/hw-vuln/l1tf.rst

@@ -445,6 +445,7 @@ The default is 'cond'. If 'l1tf=full,force' is given on the kernel command
 line, then 'always' is enforced and the kvm-intel.vmentry_l1d_flush
 module parameter is ignored and writes to the sysfs file are rejected.
 
+.. _mitigation_selection:
 
 Mitigation selection guide
 --------------------------