Add documentation on is_being_constructed and is_completed

docs/unit.rst

@@ -26,9 +26,15 @@ Unit
    .. autoattribute:: id 
    .. autoattribute:: is_alive 
    .. autoattribute:: is_being_constructed 
+      
+      Returns build_progress > 0
+
    .. autoattribute:: is_burrowed 
    .. autoattribute:: is_cloaked 
    .. autoattribute:: is_completed 
+
+      Returns build_progress >= 1
+
    .. autoattribute:: is_constructing 
    .. autoattribute:: is_flying 
    .. autoattribute:: is_idle