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.

Yeah, but companies don't get to decide how language is used. Apple can try all they want to demand that people say shit like "Use iPad to do it" or "Experience iPad" but that's just what they want and they can't stop people using English without their toxic marketing BS. Saying "Use an iPad to do it" is completely acceptable language and more in line with existing language no-matter what Apple marketing says.