batman-adv: Document optional batadv_algo_ops

Some operations in batadv_algo_ops are optional and marked as such in the
kerneldoc. But some of them miss the "(optional)" in their kerneldoc. These
have to also be marked to give an implementor of an algorithm the correct
background information without looking in the code calling these function
pointers.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>