Fix documentation regarding conversion to drbd

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

doc/admin.rst

@@ -542,7 +542,7 @@ modify`` command::
 
   # later convert it to redundant
   gnt-instance stop INSTANCE
-  gnt-instance modify -t drbd INSTANCE
+  gnt-instance modify -t drbd -n NEW_SECONDARY INSTANCE
   gnt-instance start INSTANCE
 
   # and convert it back

man/gnt-instance.sgml

@@ -1585,7 +1585,10 @@ instance5: 11225
           The <option>-t</option> option will change the disk template
           of the instance. Currently only conversions between the
           plain and drbd disk templates are supported, and the
-          instance must be stopped before attempting the conversion.
+          instance must be stopped before attempting the
+          conversion. When changing from the plain to the drbd disk
+          template, a new secondary node must be specified via
+          the <option>-n</option> option.
         </para>
 
         <para>