update disk resize instructions

Let's use --absolute because it is less error-prone: I originally
thought the number there was the *final* disk space. But actually,
without --absolute, it is *added* to the existing disk space. This can
lead to fairly catastrophic results if we add too much disk space, as
reverting this requires a complex backup / restore procedure of the
VM, because grow-disk doesn't support shrinking disks.

Also mention this problem in the docs.

howto/ganeti.md

@@ -158,11 +158,24 @@ to the `/etc/network/interfaces*` files:
 The [gnt-instance grow-disk](http://docs.ganeti.org/ganeti/2.15/man/gnt-instance.html#grow-disk) command can be used to change the size
 of the underlying device:
 
-    gnt-instance grow-disk test1.torproject.org 0 16g
+    gnt-instance grow-disk --absolute test1.torproject.org 0 16g
     gnt-instance reboot test1.torproject.org
 
 The number `0` in this context, indicates the first disk of the
-instance. Then the filesystem needs to be resized inside the VM:
+instance.  The amount specified is the final disk size (because of the
+`--absolute` flag). In the above example, the final disk size will be
+16GB. To *add* space to the existing disk, remove the `--absolute`
+flag:
+
+    gnt-instance grow-disk test1.torproject.org 0 16g
+    gnt-instance reboot test1.torproject.org
+
+In the above example, 16GB will be **ADDED** to the disk. Be careful
+with resizes, because it's not possible to revert such a change:
+`grow-disk` does support shrinking disks. The only way to revert the
+change is by exporting / importing the instance.
+
+Then the filesystem needs to be resized inside the VM:
 
     ssh root@test1.torproject.org