Explain Camera2dBundle.projection needs to be set carefully (#11115)

Encountered it while implementing
#11109.

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_core_pipeline/src/core_2d/camera_2d.rs

@@ -22,6 +22,10 @@ pub struct Camera2d;
 pub struct Camera2dBundle {
  pub camera: Camera,
  pub camera_render_graph: CameraRenderGraph,
+ /// Note: default value for `OrthographicProjection.near` is `0.0`
+ /// which makes objects on the screen plane invisible to 2D camera.
+ /// `Camera2dBundle::default()` sets `near` to negative value,
+ /// so be careful when initializing this field manually.
  pub projection: OrthographicProjection,
  pub visible_entities: VisibleEntities,
  pub frustum: Frustum,