Clarify docs on specifying multiple filters with pytest.mark.filterwarnings

[soxofaan]

Specifying multiple filterwarnings filter in the correct order can be a bit confusing.

When using @pytest.mark.filterwarnings as decorator, one has to use the reverse order compared to stdlib's warnings.filterwarnings (because of the order that decorators are evaluated).
When using pytestmark (list of pytest.mark.filterwarnings marks), it's the other way around.

I think this deserves a bit of clarification in the docs

FYI: I'm actually already working on a PR for this