Add documentation on georeferencing

src/ifcopenshell-python/ifcopenshell/util/unit.py

@@ -145,6 +145,16 @@ def convert(value, from_prefix, from_unit, to_prefix, to_unit):
     return value
 
 
+"""Returns a unit scale factor to convert to and from IFC project length units and SI meters
+
+Example::
+
+    ifc_project_length * unit_scale = si_meters
+    si_meters / unit_scale = ifc_project_length
+
+:returns: The scale factor
+:rtype: float
+"""
 def calculate_unit_scale(file):
     units = file.by_type("IfcUnitAssignment")[0]
     unit_scale = 1