batman-adv: Add kernel-doc to externally visible functions

According to the kernel-doc documentation, externally visible functions
should be documented. This refers to all all non-static function which can
(and will) be used by functions in other sources files.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>