Correct documentation regarding is_constructing

Previously it was documented as an attribute, but it is in fact method.
Describe the method, its arguments and its return value.

docs/unit.rst

@@ -48,7 +48,6 @@ Unit
 
       Returns build_progress >= 1
 
-   .. autoattribute:: is_constructing 
    .. autoattribute:: is_flying 
    .. autoattribute:: is_idle 
    .. autoattribute:: is_powered 
@@ -85,6 +84,10 @@ Unit
 
       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).
 
+   .. method:: Unit.is_constructing(self, unit_type: library.UnitType)
+
+      Returns true if the unit is currently constructing another unit of type `unit_type`. Note that `unit_type` needs to be an instance of :class:`library.UnitType`.
+
    .. automethod:: stop
    .. automethod:: attack_unit
    .. automethod:: attack_move