Update migration guide for logging changes (#1372)

Co-authored-by: Charles Bournhonesque <cbournhonesque@snapchat.com>

Author: cBournhonesque

release-content/0.14/migration-guides/12045_Add_access_to_App_within_LogPluginupdate_subscriber.md

@@ -1 +1,28 @@
-The `LogPlugin`’s `update_subscriber` field has been replaced with `custom_layer`
+The `LogPlugin`’s `update_subscriber` field has been replaced with the more flexible `custom_layer`.
+
+```rust
+// in 0.13
+fn update_subscriber(_: &mut App, subscriber: BoxedSubscriber) -> BoxedSubscriber {
+    Box::new(subscriber.with(CustomLayer))
+}
+
+LogPlugin {
+    update_subscriber: Some(update_subscriber),
+    ..default()
+}
+
+// in 0.14
+fn custom_layer(_app: &mut App) -> Option<BoxedLayer> {
+    // You can provide multiple layers like this, since Vec<Layer> is also a layer:
+    Some(Box::new(vec![
+        bevy::log::tracing_subscriber::fmt::layer()
+            .with_file(true)
+            .boxed(),
+        CustomLayer.boxed(),
+    ]))
+}
+LogPlugin {
+    custom_layer,
+    ..default()
+}
+```

release-content/0.14/migration-guides/13159_Improve_tracing_layer_customization.md

@@ -220,12 +220,6 @@ url = "https://github.com/bevyengine/bevy/pull/12164"
 areas = ["Diagnostics"]
 file_name = "12164_Make_sysinfo_diagnostic_plugin_optional.md"
 
-[[guides]]
-title = "Add access to `App` within `LogPlugin::update_subscriber`"
-url = "https://github.com/bevyengine/bevy/pull/12045"
-areas = ["Diagnostics"]
-file_name = "12045_Add_access_to_App_within_LogPluginupdate_subscriber.md"
-
 [[guides]]
 title = "Improve tracing layer customization"
 url = "https://github.com/bevyengine/bevy/pull/13159"