In certain situations, an apostrophe can be used to aid clarity. The word "dos" looks somewhat strange because it's not a word we see very often. However, since it's coupled with "don'ts" I feel that the meaning is pretty obvious even without the addition of an apostrophe.
Sometimes, an apostrophe can be avoided in situations like that by rewording a sentence or by clarifying things in another way, e.g. by using quotation marks, italics or capital letters.
Your screenshot from the Cambridge Dictionary is interesting, John (Boatbuilder). The examples of sentences have probably been taken from a range of sources. What they show is that there's no hard-and-fast rule for some constructions. What's important is to express things as clearly as possible, and to be consistent within a text.