Add more detailed explanation regarding the function of "--rdns-internal"

Add more detailed explanation regarding the function of "--rdns-internal"

As agreed in last thursday's IRC meeting [1], I've added a comment to the
code explaining "--rdns-internal".  It's really very much an internal
option, so it does not need to go to the man page.

[1] http://thread.gmane.org/gmane.network.openvpn.devel/4573

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@redhat.com>
Signed-off-by: David Sommerseth <davids@redhat.com>
(cherry picked from commit cdb3a5c0864e0fe8d0b814de1f024fd624dd3b1c)