Trigger::components() does not return the expected results

## How can Bevy's documentation be improved?

The docs [here](https://github.com/bevyengine/bevy/blob/9bc0ae33c37728fc5215c833e3f4e78dc2abb7b9/crates/bevy_ecs/src/observer/mod.rs#L90) mention that the trigger components will be
```
the components that triggered the observer, out of the
/// components defined in `B`
```

That is not the observed behaviour
```
world.spawn(
Observer::new(|trigger: Trigger| {
dbg!(trigger.components());
})
);
let mut entity = world.spawn((A, B));
```

When the observer runs, the `trigger.components()` will contain the component ids for A and B.

Read the observer documentation in crates/bevy_ecs/src/observer/mod.rs around the linked section, then run the provided Observer::new and world.spawn((A, B)) reproducer. Compare the documented Trigger::components() behavior with the observed component IDs and update the documentation so it accurately describes the result.

Written by the indexing model from the issue text.