Expanded rdoc about primary keys being protected from mass assignment.

People need to make sure they are generated internally or added to attr_accessible so they can be mass assigned (for instance, from an import job).

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -44,7 +44,8 @@ def dangerous_attribute_method?(method_name)
         end
 
         # Defines the primary key field -- can be overridden in subclasses. Overwriting will negate any effect of the
-        # primary_key_prefix_type setting, though.
+        # primary_key_prefix_type setting, though. Since primary keys are usually protected from mass assignment,
+        # remember to let your database generate them or include the key in +attr_accessible+.
         def primary_key
           @primary_key = reset_primary_key unless defined? @primary_key
           @primary_key