Document difference better on what _some and _none do and when they're applyed.

In issue https://github.com/directus/directus/issues/18863 a lot of confusion has came from that we documented that in a relational filter, by default `_some` is always applied. This is not the case and should be corrected on when/why you need it and when/why not.

I'm currently too out of topic to remember the exact details and would have to look into it again so please reach out to me when this issue gets picked up so I can provide all the details and assist with making the docs more accurate.