Improve documentation for enum reflection

[MrGVSV]

How can Bevy's documentation be improved?

Enums require all variant fields to implement FromReflect in order to implement Reflect themselves (this is due to a similar limitation as Vec<T> in the Reflect::apply method implementation).

This behavior should either be documented on Reflect or at least on Enum.

[MrGVSV]

This should be covered by #7148