Foo and bar aren’t semantic or particularly helpful. They’re not actual words, and are particularly confusing to minorities and non-native English speakers. At some point, people involved in the software sector learn that they’re just placeholders, but the time it takes to realize this can vary and is always > 0.
I also see baz and bim(?) in the spec. The use of foo and bar in some examples, but baz and bim in others, will plausibly make some people think there is a reason for the variance – that baz and bim might mean something different. After all, none of these are actual English words, and learners (esp people in other countries) might assume they’re terms of art in Markdown or the software industry. (By the way, what is bim?)
Lastly, these one-word examples could do with being replaced by phrases and sentences, since the latter is the more common scenario for Markdown. Simple grade school stuff like “Jack and Jill…” or lines from Dr. Seuss would be less distracting than foo, bar, etc, and extremely unlikely to be confused for meaningful keywords.