Document that AVA 4 cannot be run globally

[binyamin]

In commit a763592, the docs were modified to explain that AVA 4 cannot be run globally. In f09742f, this was removed (diff). I assume the second change was a mistake. In any case, the user should know about this detail.

[novemberborn]

Sure, we could add a "Note: AVA 4 cannot be run globally." PR welcome.

[binyamin]

@novemberborn how about in /docs/08-common-pitfalls.md?

[novemberborn]

@binyamin sure, sounds good.