"If one array is shorter than the other, use the optional third parameter (defaulted to 0) to or the unmatched elements."

I assumed it had to be used once, and then (if necessary) pad with 0s.

And what gave you that idea?

If one array is shorter than the other, use the optional second parameter (defaulted to 0) to OR the unmatched elements.

Could you suggest a better wording?

5. Second reinforcement -> "cccaa"

Nope, it will be "ccbaa" because the third b hasn't been "used" yet