Bug Fixes:
- Fixed wrong check added at e7b654a (bugreport:9095)
- Spirit sphere reduces damage for skill hit num < 0, eg. MO_TRIPLEATTACK has hit num -3. (bugreport:9094)
Misc:
- Removed unused parameters in battle_calc_bg_damage() and battle_calc_gvg_damage()
- Some random src docs.

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>