[skip ci] Document id_value method

Adds documention for ActiveRecord::ModelSchema#id_value as a utility
when using composite primary keys.

Author: gmcgibbon

activerecord/lib/active_record/model_schema.rb

@@ -6,6 +6,13 @@ module ActiveRecord
   module ModelSchema
     extend ActiveSupport::Concern
 
+    ##
+    # :method: id_value
+    # :call-seq: id_valiue
+    #
+    # Returns the underlying column value for a column named "id". Useful when defining
+    # a composite primary key including an "id" column so that the value is readable.
+
     ##
     # :singleton-method: primary_key_prefix_type
     # :call-seq: primary_key_prefix_type