IIUC, that seems to be saying "don't write sentences in this structure" rather than "here's how it would look if you did write it". (Though it would seem strange for a manual to dictate the grammar for its own name.)
Following the example of Big Mac sandwiches (not Big Macs) I believe you'd write McDonald's restaurants' (not McDonald's'). I.E. always keep the trademarked word as-is, add sandwich or other nouns where needed to make the grammar work.
I think the confusion in this thread and elsewhere makes it clear why companies release brand guides.
I think the confusion in this thread and elsewhere makes it clear why companies release brand guides.