batman-adv: fix batadv_skb_is_frag() kernel-doc

batman-adv: fix batadv_skb_is_frag() kernel-doc

The kernel-doc comment for batadv_skb_is_frag() contained two errors:

* the function description referred to "gain a unicast packet" instead
  of "contains unicast fragment".
* the Return section omitted "merged" from "newly skb", leaving the
  description grammatically incorrect and inconsistent with the
  function description.

Fixes: bc62216dc8e2 ("batman-adv: frag: disallow unicast fragment in fragment")
Signed-off-by: Sven Eckelmann <sven@narfation.org>