Detailed migration guide for automatic reflection registration (#21109)

## Objective

- The migration for automatic registration is non-trivial with default
features off.
- Fixes #21092.

## Solution

- Carefully explain how this works and the steps users and libraries
should take to migrate.
- Link #15030 (auto-register), #20345 (registration removal) and #20893
(example of generic registration) to guide users who want to see this in
action.

---------

Co-authored-by: Chris Biscardi <[email protected]>

Author: 2 people

release-content/migration-guides/reflect_registration_changes.md

@@ -0,0 +1,27 @@
+---
+title: Changes to type registration for reflection
+pull_requests: [15030, 20435, 20893]
+---
+
+Calling `.register_type` has long been a nuisance for Bevy users: both library authors and end users.
+This step was previously required in order to register reflected type information in the `TypeRegistry`.
+
+In Bevy 0.17 however, types which implement `Reflect` are now automatically registered, with the help of some compiler magic.
+You should be able to remove almost all of your `register_type` calls.
+This comes with a few caveats however:
+
+1. Automatic type registration is gated by feature flags.
+2. There are two approaches to do this: one has incomplete platform support, while the other relies on a specific project structure.
+3. Generic types are not automatically registered, and must still be manually registered.
+
+In order for Bevy to automatically register your types, you need to turn on the `reflect_auto_register` feature, or the fallback `reflect_auto_register_static`.
+The `reflect_auto_register` feature is part of Bevy's default features, and can be overridden by the `reflect_auto_register_static` feature flag.
+Be aware that the `reflect_auto_register_static` feature comes with some caveats for project structure: check the docs for [load_type_registrations!](https://docs.rs/bevy/0.17.0-rc.1/bevy/reflect/macro.load_type_registrations.html) and follow the [`auto_register_static` example](https://github.com/bevyengine/bevy/tree/main/examples/reflection/auto_register_static).
+
+We recommend that you:
+
+1. Enable `reflect_auto_register` in your application code, CI and in examples/tests. You can enable `bevy` features for tests only by adding a matching copy of `bevy` to `dev-dependencies` with the needed features enabled.
+2. Do not enable the `reflect_auto_register` feature or the fallback `reflect_auto_register_static` in your library code.
+3. As a library author, you can safely remove all non-generic `.register_type` calls.
+4. As a user, if you run into an unregistered generic type with the correct feature enabled, file a bug with the project that defined the offending type, and workaround it by calling `.register_type` manually.
+5. If you are on an unsupported platform but need reflection support, try the `reflect_autoregister_static` feature, and consider working upstream to add support for your platform in `inventory`. As a last resort, you can still manually register all of the needed types in your application code.