Describe price quantities.

docs/model/type/quantity.md

@@ -18,6 +18,7 @@ their module is enabled. The modules included with farmOS define the following
 Quantity types:
 
 - Standard
+- Price
 
 ## ID
 
@@ -95,3 +96,11 @@ above, some types add additional type-specific fields. These include:
 #### Standard Quantities
 
 Standard Quantities do not define any type-specific fields.
+
+#### Price Quantities
+
+Price Quantities have additional "Unit Price" and "Unit Quantity" attributes,
+alongside the standard "Value" attribute. When these are used, the "Value"
+attribute is considered to be the total value ("Unit Price" multiplied by
+"Unit Quantity"). The "Units" of a Price Quantity will generally be a
+currency code such as "USD" or "EUR".