Ambiguous `BuildChildren` documentation

[ryanbai1412]

How can Bevy's documentation be improved?

To me, the BuildChildren documentation is somewhat unclear.

At the moment, I can't tell if each of the methods updates the Parent component of the relevant children. For example, reading the documentation of add_child, I can tell that the Children component of the relevant parent entities will get updated, but I can't tell what happens with the Parent component of child.