Add migration guide for reflect attribute

Author: dloukadakis

release-content/migration-guides/reflect_parenthesis.md

@@ -0,0 +1,25 @@
+---
+title: "`#[reflect(...)] now supports only parenthesis`" 
+pull_requests: [21400]
+---
+
+Previously, the `#[reflect(...)]` attribute of the `Reflect` derive macro
+supported parenthesis, braces, or brackets. Now it supports only parenthesis.
+
+```rust
+/// before
+#[derive(Clone, Reflect)
+#[reflect[Clone]]
+
+/// after
+#[derive(Clone, Reflect)
+#[reflect(Clone)]
+
+/// before
+#[derive(Clone, Reflect)
+#[reflect{Clone}]
+
+/// after
+#[derive(Clone, Reflect)
+#[reflect(Clone)]
+```