Explain usage of prepass shaders in docs for Material trait (bevyen…

…gine#9025)

# Objective

- Fixes bevyengine#8696.

## Solution

- Added a paragraph describing the usage of the `prepass_vertex_shader`
and `prepass_fragment_shader`.

crates/bevy_pbr/src/material.rs

@@ -136,12 +136,18 @@ pub trait Material: Asset + AsBindGroup + Clone + Sized {
 
     /// Returns this material's prepass vertex shader. If [`ShaderRef::Default`] is returned, the default prepass vertex shader
     /// will be used.
+    ///
+    /// This is used for the various [prepasses](bevy_core_pipeline::prepass) as well as for generating the depth maps
+    /// required for shadow mapping.
     fn prepass_vertex_shader() -> ShaderRef {
         ShaderRef::Default
     }
 
     /// Returns this material's prepass fragment shader. If [`ShaderRef::Default`] is returned, the default prepass fragment shader
     /// will be used.
+    ///
+    /// This is used for the various [prepasses](bevy_core_pipeline::prepass) as well as for generating the depth maps
+    /// required for shadow mapping.
     #[allow(unused_variables)]
     fn prepass_fragment_shader() -> ShaderRef {
         ShaderRef::Default