Explain that optionalTableField uses DEFAULT when you provide Nothing

src/Opaleye/Internal/Table.hs

@@ -93,7 +93,9 @@ requiredTableField :: String -> TableFields (Field_ n a) (Field_ n a)
 requiredTableField = lmap Just . optionalTableField
 
 -- | 'optionalTableField' is for fields that you can omit on writes,
--- such as fields which have defaults or which are SERIAL.
+-- such as fields which have defaults or which are SERIAL.  Setting
+-- the write value to @Nothing@ uses SQL @DEFAULT@ in the generated
+-- update.
 optionalTableField :: String -> TableFields (Maybe (Field_ n a)) (Field_ n a)
 optionalTableField columnName = TableFields
   (optionalW columnName)