add doc comment to manual texture view and handle (#21087)

# Objective
Addresses #21037


## Solution

explain how `ManualTextureViews` can be used.

------

I think the handles should be atomically increasing ids like other
internal ids so that users don't have to pick a random offset into `u32`
to avoid colliding with other `ManualTextureView`s (I'm not sure if
there ever are multiple systems/libraries using this functionality in
the same world, but it's not forbidden).
A quick search through github shows most people just start at 0.

I'm hoping these docs might make it into 0.17 though, so unless someone
disagrees with me or that change would prevent some use-case I'm not
aware of, I plan on creating a PR for that in in 0.18.

Author: janis-bhm

crates/bevy_camera/src/camera.rs

@@ -882,6 +882,8 @@ pub enum NormalizedRenderTarget {
 }
 
 /// A unique id that corresponds to a specific `ManualTextureView` in the `ManualTextureViews` collection.
+///
+/// See `ManualTextureViews` in `bevy_camera` for more details.
 #[derive(Default, Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Component, Reflect)]
 #[reflect(Component, Default, Debug, PartialEq, Hash, Clone)]
 pub struct ManualTextureViewHandle(pub u32);

crates/bevy_render/src/texture/manual_texture_view.rs

@@ -26,7 +26,30 @@ impl ManualTextureView {
     }
 }
 
-/// Stores manually managed [`ManualTextureView`]s for use as a [`bevy_camera::RenderTarget`].
+/// Resource that stores manually managed [`ManualTextureView`]s for use as a [`RenderTarget`](bevy_camera::RenderTarget).
+/// This type dereferences to a `HashMap<ManualTextureViewHandle, ManualTextureView>`.
+/// To add a new texture view, pick a new [`ManualTextureViewHandle`] and insert it into the map.
+/// Then, to render to the view, set a [`Camera`](bevy_camera::Camera)s `target` to `RenderTarget::TextureView(handle)`.
+/// ```ignore
+/// # use bevy_ecs::prelude::*;
+/// # let mut world = World::default();
+/// # world.insert_resource(ManualTextureViews::default());
+/// # let texture_view = todo!();
+/// let manual_views = world.resource_mut::<ManualTextureViews>();
+/// let manual_view = ManualTextureView::with_default_format(texture_view, UVec2::new(1024, 1024));
+///
+/// // Choose an unused handle value; it's likely only you are inserting manual views.
+/// const MANUAL_VIEW_HANDLE: ManualTextureViewHandle = ManualTextureViewHandle::new(42);
+/// manual_views.insert(MANUAL_VIEW_HANDLE, manual_view);
+///
+/// // Now you can spawn a Cemera that renders to the manual view:
+/// # use bevy_camera::{Camera, RenderTarget};
+/// world.spawn(Camera {
+///     target: RenderTarget::TextureView(MANUAL_VIEW_HANDLE),
+///     ..Default::default()
+/// });
+/// ```
+/// Bevy will then use the `ManualTextureViews` resource to find your texture view and render to it.
 #[derive(Default, Clone, Resource, ExtractResource)]
 pub struct ManualTextureViews(HashMap<ManualTextureViewHandle, ManualTextureView>);