ssr fix and add some documents (#21877)

# Objective
Screen space reflection didn't work for my program, so I tried cloning
bevy repo and running the ssr example to see whether it works or not,
and it didn't. I figured out that the problem is that
`prepare_ssr_pipelines` system only runs for views with
`ExtractedAtmosphere`. So I removed this filter because it already uses
`Has<ExtractedAtmosphere>`. And the example works afterwards.

I later found that the `With<ExtractedAtmosphere>` filter is not added
to bevy 0.17, so the problem in my own program is not the same as the
ssr example. I compared the ssr example and my program and found that I
didn't add `DefaultOpaqueRendererMethod::deferred()` resource. So I
added a few docs to state that deferred rendering is not automatically
enabled when ssr is added.


## Solution
1. remove `With<ExtractedAtmosphere>` filter in `prepare_ssr_pipelines`.
2. add more documentation.

## Testing
I tested the ssr example and atmosphere example and both look fine.

Author: newDINO

crates/bevy_pbr/src/ssr/mod.rs

@@ -62,7 +62,8 @@ pub struct ScreenSpaceReflectionsPlugin;
 ///
 /// Screen-space reflections currently require deferred rendering in order to
 /// appear. Therefore, they also need the [`DepthPrepass`] and [`DeferredPrepass`]
-/// components, which are inserted automatically.
+/// components, which are inserted automatically,
+/// but deferred rendering itself is not automatically enabled.
 ///
 /// SSR currently performs no roughness filtering for glossy reflections, so
 /// only very smooth surfaces will reflect objects in screen space. You can
@@ -428,7 +429,6 @@ pub fn prepare_ssr_pipelines(
             With<ScreenSpaceReflectionsUniform>,
             With<DepthPrepass>,
             With<DeferredPrepass>,
-            With<ExtractedAtmosphere>,
         ),
     >,
 ) {