First you need to activate Developer menu.
File - Options - Customize Ribbon - (On the right part of the window select check-mark on Developer tab) - Ok
When Developer menu is there, you need to select Rich Text Content Control (Aa) and then put that control in the desired place in the document.
Then do the restriction, but make sure you allow Editing forms (see screenshot).
This will make only that area editable. PM me for sample file.
There is only one disadvantage that I have found - you can not edit some elements, but you can copy them from other non-restricted document.