DOC: Explain units

src/doc/cfood.rst

@@ -154,6 +154,28 @@ create lists or multi properties instead of single values:
 			 *Measurement #  Multi Property (properties are removed before run)
 			 Measurement  #  Overwrite
 
+Values and units
+----------------
+
+Units of numeric values can be set by providing a property value not
+as a single string, but as a dictionary with a ``value`` and a
+``unit`` key. Within a converter definition this could look the
+following.
+
+.. code-block:: yaml
+
+   ValueWithUnitElt:
+     type: TextElement
+     match_name: ^my_prop$
+     match_value: "^(?P<number>\d+\.?\d*)\s+(?P<unit>.+)"  # Extract value and unit from a string which
+							   # has a number followed by at least one whitespace
+							   # character followed by a unit.
+     records:
+       MyRecord:
+	 MyProp:
+	   value: $number
+	   unit: $unit
+
 
 File Entities
 -------------