Improve "on_unit_took_damage" docstring

[BurnySc2]

Developers that rarely play SC2 should be made aware of the exceptions that would want to be handled, such as: Stimming, transforming units into having less HP (hellbat -> hellion), abilities (viper consume), terran structures burning down, zerg structures being off creep, perhaps splash damage.

Location of docstring to be edited: https://github.com/BurnySc2/python-sc2/blob/40dc00a01544d1405ad15b25ae6250a2e9382ec9/sc2/bot_ai.py#L1271