Improve documentation (TheBevyFlock#175)

Co-authored-by: Chris Biscardi <[email protected]>
Co-authored-by: Ben Frankel <[email protected]>

src/lib.rs

@@ -78,6 +78,12 @@ fn spawn_camera(mut commands: Commands) {
     commands.spawn((
         Name::new("Camera"),
         Camera2dBundle::default(),
+        // Render all UI to this camera.
+        // Not strictly necessary since we only use one camera,
+        // but if we don't use this component, our UI will disappear as soon
+        // as we add another camera. This includes indirect ways of adding cameras like using
+        // [ui node outlines](https://bevyengine.org/news/bevy-0-14/#ui-node-outline-gizmos)
+        // for debugging. So it's good to have this here for future-proofing.
         IsDefaultUiCamera,
     ));
 }

src/ui/widgets.rs

@@ -144,6 +144,9 @@ impl Containers for Commands<'_, '_> {
 }
 
 /// An internal trait for types that can spawn entities.
+/// This is here so that [`Widgets`] can be implemented on all types that
+/// are able to spawn entities.
+/// Ideally, this trait should be [part of Bevy itself](https://github.com/bevyengine/bevy/issues/14231).
 trait Spawn {
     fn spawn<B: Bundle>(&mut self, bundle: B) -> EntityCommands;
 }