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:

python-sc2/sc2/bot_ai.py

Line 1271 in 40dc00a

async def on_unit_took_damage(self, unit: Unit, amount_damage_taken: float):