Adding some docs for archetype internals (#12578)

# Objective


I was reading some of the Archetype and Bundle code and was getting
confused a little bit in some places (is the `archetype_id` in
`AddBundle` the source or the target archetype id?).
Small PR that adds some docstrings to make it easier for first-time
readers.

crates/bevy_ecs/src/archetype.rs

@@ -114,7 +114,10 @@ pub(crate) enum ComponentStatus {
 }
 
 pub(crate) struct AddBundle {
+    /// The target archetype after the bundle is added to the source archetype
     pub archetype_id: ArchetypeId,
+    /// For each component iterated in the same order as the source [`Bundle`](crate::bundle::Bundle),
+    /// indicate if the component is newly added to the target archetype or if it already existed
     pub bundle_status: Vec<ComponentStatus>,
 }
 
@@ -285,8 +288,12 @@ impl ArchetypeEntity {
     }
 }
 
+/// Internal metadata for an [`Entity`] getting removed from an [`Archetype`].
 pub(crate) struct ArchetypeSwapRemoveResult {
+    /// If the [`Entity`] was not the last in the [`Archetype`], it gets removed by swapping it out
+    /// with the last entity in the archetype. In that case, this field contains the swapped entity.
     pub(crate) swapped_entity: Option<Entity>,
+    /// The [`TableRow`] where the removed entity's components are stored.
     pub(crate) table_row: TableRow,
 }
 

crates/bevy_ecs/src/world/mod.rs

@@ -1836,7 +1836,7 @@ impl World {
     }
 
     /// # Panics
-    /// panics if `component_id` is not registered in this world
+    /// Panics if `component_id` is not registered in this world
     #[inline]
     pub(crate) fn initialize_non_send_internal(
         &mut self,