DefaultPlugins should mention that `ScheduleRunnerPlugin` is not always included

[VincentWo]

On the doc page of DefaultPlugins it is stated that it includes ScheduleRunnerPlugin, but the doc page 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 :)

[alice-i-cecile]

Yep that sounds great!