Add default disk template selection to man pages

This documents the newly implemented behavior of picking
the disk template from the list of enabled disk templates
on instance creation.
Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>

man/gnt-cluster.rst

@@ -557,7 +557,9 @@ this list, see **gnt-instance**\(8). Note that in contrast to the list of
 disk templates in the ipolicy, this list is a hard restriction. It is not
 possible to create instances with disk templates that are not enabled in
 the cluster. It is also not possible to disable a disk template when there
-are still instances using it.
+are still instances using it. The first disk template in the list of
+enabled disk template is the default disk template. It will be used for
+instance creation, if no disk template is requested explicitely.
 
 MASTER-FAILOVER
 ~~~~~~~~~~~~~~~

man/gnt-instance.rst

@@ -763,7 +763,10 @@ don't need to pass them with the ``-n`` option. For more information
 please refer to the instance allocator documentation.
 
 The ``-t (--disk-template)`` options specifies the disk layout type
-for the instance.  The available choices are:
+for the instance. If no disk template is specified, the default disk
+template is used. The default disk template is the first in the list
+of enabled disk templates, which can be adjusted cluster-wide with
+``gnt-cluster modify``. The available choices for disk templates are:
 
 diskless
     This creates an instance with no disks. Its useful for testing only