Fix maxTaskFailures comment

If maxTaskFailures is 1, the task set is aborted after 1 task failure. Other documentation and the code supports this reading, I think it's just this comment that was off. It's easy to make this mistake — can you please double-check if I'm correct? Thanks!

Author: Daniel Darabos <darabos.daniel@gmail.com>

Closes #6621 from darabos/patch-2 and squashes the following commits:

dfebdec [Daniel Darabos] Fix comment.