Update the doc comments for UiCameraMap and UiCameraMapper. (#20521)

# Objective

The docs comments for `UiCameraMap` and `UiCameraMapper` are out of date
and incorrectly refer to a default camera and `UiTargetCamera` that
aren't present.

## Solution

Update the doc comments

Author: ickshonpe

crates/bevy_ui_render/src/lib.rs

@@ -301,7 +301,9 @@ pub struct UiCameraMap<'w, 's> {
 }
 
 impl<'w, 's> UiCameraMap<'w, 's> {
-    /// Get the default camera and create the mapper
+    /// Creates a [`UiCameraMapper`] for performing repeated camera-to-render-entity lookups.
+    ///
+    /// The last successful mapping is cached to avoid redundant queries.
     pub fn get_mapper(&'w self) -> UiCameraMapper<'w, 's> {
         UiCameraMapper {
             mapping: &self.mapping,
@@ -311,14 +313,18 @@ impl<'w, 's> UiCameraMap<'w, 's> {
     }
 }
 
+/// Helper for mapping UI target camera entities to their corresponding render entities,
+/// with caching to avoid repeated lookups for the same camera.
 pub struct UiCameraMapper<'w, 's> {
     mapping: &'w Query<'w, 's, RenderEntity>,
+    /// Cached camera entity from the last successful `map` call.
     camera_entity: Entity,
+    /// Cached camera entity from the last successful `map` call.
     render_entity: Entity,
 }
 
 impl<'w, 's> UiCameraMapper<'w, 's> {
-    /// Returns the render entity corresponding to the given `UiTargetCamera` or the default camera if `None`.
+    /// Returns the render entity corresponding to the given [`ComputedUiTargetCamera`]'s camera, or none if no corresponding entity was found.
     pub fn map(&mut self, computed_target: &ComputedUiTargetCamera) -> Option<Entity> {
         let camera_entity = computed_target.camera()?;
         if self.camera_entity != camera_entity {
@@ -330,6 +336,7 @@ impl<'w, 's> UiCameraMapper<'w, 's> {
         Some(self.render_entity)
     }
 
+    /// Returns the cached camera entity from the last successful `map` call.
     pub fn current_camera(&self) -> Entity {
         self.camera_entity
     }