batman-adv: update kernel doc of batadv_tt_global_del_orig_entry()

The updated kernel doc & additional comment shall prevent accidental
copy & paste errors or calling the function without the required
precautions.

Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>