Document Unit method ability

docs/unit.rst

@@ -66,6 +66,10 @@ Unit
 
    Methods:
 
+   .. automethod:: ability
+
+      Call an ability directly, different abilities has different targets. Some target the unit itself (no argument), target a point (Point2D as argument) and some target a Unit (instance of Unit as argument).
+
    .. automethod:: stop
    .. automethod:: attack_unit
    .. automethod:: attack_move