DefaultPlugins should mention that `ScheduleRunnerPlugin` is not always included

On the [doc page](https://docs.rs/bevy/0.15.3/bevy/struct.DefaultPlugins.html) of `DefaultPlugins` it is stated that it includes `ScheduleRunnerPlugin`, but the [doc page](https://docs.rs/bevy/0.15.3/bevy/app/struct.ScheduleRunnerPlugin.html) of `ScheduleRunnerPlugin` says that it is not included.

Both are kind of correct, kind of wrong: `ScheduleRunnerPlugin` is currently included in `DefaultPlugins` if and only if the feature `bevy_window` is **not** enabled. I would suggest to clarify this.

I can gladly fix this myself, just wanted to check beforehand because I would have to fiddle a bit with the `plugin_group` macro :)