Observe should state that Trigger<_> must be first arg

[torsteingrindvik]

How can Bevy's documentation be improved?

Observe

There is a limitation that Trigger<_> must be the first arg.

So currently when doing:

App::new().observe(f);

then:

/// works
fn foo(trigger: Trigger<Foo>, mut commands: Commands) {}

/// does not
fn bar(mut commands: Commands, trigger: Trigger<Foo>) {}

this should be documented.

[alice-i-cecile]

We should ensure that the same note is present for exclusive and pipe systems while we're at it.