add migration guide

Author: ecoskey

release-content/migration-guides/render_target_info_error.md

@@ -0,0 +1,11 @@
+---
+title: "RenderTarget error handling"
+pull_requests: [20503]
+---
+
+`NormalizedRenderTargetExt::get_render_target_info` now returns a `Result`,
+with the `Err` variant indicating which render target (image, window, etc)
+failed to load its metadata.
+
+This should mostly be treated as a hard error, since it indicates the rendering
+state of the app is broken.