add some model api docs

app/models/article_price.rb

@@ -1,21 +1,41 @@
 class ArticlePrice < ActiveRecord::Base
 
+  # @!attribute price
+  #   @return [Number] Net price
+  #   @see Article#price
+  # @!attribute tax
+  #   @return [Number] VAT percentage
+  #   @see Article#tax
+  # @!attribute deposit
+  #   @return [Number] Deposit
+  #   @see Article#deposit
+  # @!attribute unit_quantity
+  #   @return [Number] Number of units in wholesale package (box).
+  #   @see Article#unit
+  #   @see Article#unit_quantity
+  # @!attribute article
+  #   @return [Article] Article this price is about.
   belongs_to :article
+  # @!attribute order_articles
+  #   @return [Array<OrderArticle>] Order articles this price is associated with.
   has_many :order_articles
 
+  localize_input_of :price, :tax, :deposit
+
   validates_presence_of :price, :tax, :deposit, :unit_quantity
   validates_numericality_of :price, :greater_than_or_equal_to => 0
   validates_numericality_of :unit_quantity, :greater_than => 0
   validates_numericality_of :deposit, :tax
 
-  localize_input_of :price, :tax, :deposit
-
-  # The financial gross, net plus tax and deposit.
+  # Gross price = net price + deposit + tax.
+  # @return [Number] Gross price.
+  # @todo remove code-duplication with Article
   def gross_price
     ((price + deposit) * (tax / 100 + 1)).round(2)
   end
 
-  # The price for the foodcoop-member.
+  # @return [Number] Price for the foodcoop-member.
+  # @todo remove code-duplication with Article
   def fc_price
     (gross_price  * (FoodsoftConfig[:price_markup] / 100 + 1)).round(2)
   end