Staging: batman-adv: Add information about batman-adv sysfs entries

Add documents about new sysfs entries in
 * /sys/class/net/<iface>/batman-adv/mesh_iface
 * /sys/class/net/<mesh_iface>/mesh/vis_mode

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>