Document the origin of unit.id=0. closes #112.

Author: GraylinKim

sc2reader/data/__init__.py

@@ -44,7 +44,9 @@ def __init__(self, unit_id, flags):
         #: A reference to the player that killed this unit. Not always available.
         self.killed_by = None
 
-        #: The unique in-game id for this unit
+        #: The unique in-game id for this unit. The id can sometimes be zero because
+        #: TargetAbilityEvents will create a new unit with id zero when a unit
+        #: behind the fog of war is targetted.
         self.id = unit_id
 
         self.flags = flags

sc2reader/events/game.py

@@ -155,6 +155,7 @@ def __init__(self, frame, pid, data):
             self.target_unit_type = ability_type_data.get('unit_link',None)
 
             #: Integer player id of the controlling player. Available for TargetUnit type events starting in 19595.
+            #: When the targetted unit is under fog of war this id is zero.
             self.control_player_id = ability_type_data.get('control_player_id',None)
 
             #: Integer player id of the player paying upkeep. Available for TargetUnit type events.