v2v: docs: State that --vddk-thumbprint is only required if --vddk pr…

…esent (RHBZ#1513884).

Thanks: Ming Xie, Pino Toscano.

See also:
https://bugzilla.redhat.com/show_bug.cgi?id=1513884#c2

v2v/virt-v2v.pod

@@ -660,7 +660,8 @@ See L</INPUT FROM VDDK> below for details.
 
 When using VDDK mode, these options are passed unmodified to the
 L<nbdkit(1)> VDDK plugin.  Please refer to L<nbdkit-vddk-plugin(1)>.
-Only I<--vddk-thumbprint> is required, the others are optional.
+If I<--vddk> is present, I<--vddk-thumbprint> is also required,
+the rest are optional.
 
 =item B<--vdsm-compat=0.10>