WordPress custom post type archive with description

I have to do similar things all the time. You are already using ACF, which is what I use, actually I use ACF5 Pro version now, but the same thing can be achieved using ACF4 and the Options Page add on.

What I do is create an options page for each post type and set the parent of is so it appears as part of the fly out for the post type. Then I create custom fields for the option page and one of the fields is a wysiwyg field for the content.

Each options page needs to have unique field names because ACF stores data for these fields in the wp_options table, so it means duplicating the field group and giving each field a unique name. If you are using ACF Pro, I have actually created a plugin that will actually do create the options pages through a user interface and can also do the field group duplication and updates automatically. https://github.com/Hube2/acf-options-page-adder.