Mention `QueryItem` in `WorldQueryGats`

[nicopap]

How can Bevy's documentation be improved?

QueryItem can be extremely useful, but is not very discoverable, leading to questions like this. Note that this is not the first time such a question is asked.

A good first step in making QueryItem more discoverable is mentioning it in the documentation of other items, such as WorldQueryGats.

[afonsolage]

I was using <<T as WorldQueryGats<'w>>::Fetch as Fetch<'w>>::Item until I read this issue, so yeah, I second this.