bsn: improve docs and add tests

crates/bevy_ecs/src/template.rs

@@ -416,6 +416,8 @@ impl<T: Clone + Default + Unpin> FromTemplate for T {
 pub trait SpecializeFromTemplate: Sized {}
 
 /// A [`Template`] reference to an [`Entity`].
+///
+/// This is only valid during scene spawning and should **never** be used as a [`Component`](bevy_ecs::prelude::Component) field.
 #[derive(Copy, Clone, Default, Debug)]
 pub enum EntityTemplate {
     /// A reference to a specific [`Entity`]