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.
It is a regular, but not universal, occurrence for people to elide the extra possessive 's when the word already ends in s.
For example, although adding a possessive suffix 's to the nane "Jordan" would result in "Jordan's," adding it to "James" could result in "James'". There's an apostrophe written at the end, but it's pronounced the same way as "James."
I said it's not universal because I still know people who would say "James's" (with an extra syllable at the end) in everyday speech anyway. I don't know to what extent this varies by dialect.
It's also elided when a plural already ends in s (and I think this is universal, but I haven't looked it up). For example, "the doctor's computer" (the computer of one doctor) sounds the same as "the doctors' computers" (the computers owned by the doctors). The apostrophe is written on the other side of the s, but it also sounds the same. Note that not all plurals end in s, e.g. man/men, woman/women, goose/geese, etc., and in these cases you still add the s, e.g. "the geese's beaks".
https://issuu.com/lukaszkulakowski/docs/002725