There are few things you can do in this case, as you point out-
You can leave a comment under the answer, letting the owner of that answer know that there is/are cases that have been missed. The owner of the answer can then choose to edit it in, or ignore it. The downside to this is that comments are ephemeral, and can disappear at any time.
This is a little different situation. As can be seen by a SE meta question, and this from the top voted answer:
You edit to make things better, clearer, more effective -- never to change meaning.
As Valorum points out in his comment, adding additional supporting information or cases is fine, as long as it doesn't substantially change the essence of the answer. In that case, there is the third option:
Post a new answer
I would do this in a couple of cases - You've left a comment, and the original answerer has not incorporated it in a reasonable amount of time (Or, if you look at their profile and they haven't been back to the site in an extended period of time).
Or, (and this would be the case with your answer), the new information + supporting material is longer/better/much clearer than the original answer. If you can provide that much information, then it is no longer really a list item addition, but a stand alone candidate. In addition, providing an answer of that quality also improves the site.