Could you highlight the use of vec-last and vec-sep in docs?

[mkalamarz]

I feel like it took me way too long to stumble opon these two in cli_vec(). Shouldn't this be mentioned in here or there? I was fortunate to find this info by chance when looking at the truncation example on the inline markup article (which isn't mentioned in "Building a semantic CLI" one) and thinking "Maybe I should check this cli_vec() thingy?".