doc: nextTick is replaced when domains are in use

Probably it also makes sense to document this behavior not only for `domain` module, but for `nextTick` function as well. It's very tricky and people, especially library authors, should be warned about such side effect. My further propose would be to to make a wrapper for `nextTick` inside `node.js` and make sure the actual reference is always the same so people don't get caught when they cache it.