batman-adv: Add kernel-doc to functions in headers

Externally visible functions should be documented with kernel-doc. This
usually refers to non-static functions but also static inline files in
headers are visible in other files and should therefore be documented.

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