Make docs about expect_next_instance_of consistent
What does this MR do?
It was brought up that the doc is comparing allow_any_instance_of with expect_next_instance_of, which is indeed weird. This makes it consistent that we compare expect_any_instance_of with expect_next_instance_of, but we still mention allow_any_instance_of in case people searching the source can't find it.
What are the relevant issue numbers?
In https://gitlab.com/gitlab-org/gitlab-ce/issues/54576 we want to also add allow_next_instance_of, but we haven't done that yet. We can update the docs again when we actually implemented this.