Explain where rendering is

crates/bevy_app/src/main_schedule.rs

@@ -23,6 +23,17 @@ use bevy_ecs::{
 /// * [`Update`]
 /// * [`PostUpdate`]
 /// * [`Last`]
+///
+/// # Rendering
+///
+/// Note rendering is not executed in the main schedule by default.
+/// Instead, rendering is performed in a separate [`SubApp`](crate::app::SubApp)
+/// which exchanges data with the main app in between the main schedule runs.
+///
+/// When pipelined rendering is enabled, rendering is performed in a separate thread,
+/// concurrently with the main schedule, one rendering iteration for each main schedule iteration.
+///
+/// See `RenderPlugin` and `PipelinedRenderingPlugin` for more details.
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Main;