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

Encountered it while implementing
#11109.

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

Author: stepancheg

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,