Update relationships migration guide one last time

[alice-i-cecile]

          If we merge this for 0.16, you'll need to go in and edit the 0.16 migration guide for relationships over on the bevy-website repo.

Originally posted by @alice-i-cecile in bevyengine/bevy#18699 (comment)

[Freyja-moth]

Just to add to this, we currently only mention one to many relationships and don't call out how to create custom relationships (as far as I can tell), which I feel like we should be doing.

[alice-i-cecile]

In some ways that's a "release notes" thing, since this is new functionality. On the other hand, many users likely have their own jank forms of relations in place and should migrate.