Flesh out migration guide and mark LegacyTypeResolver as deprecated

Author: DerManoMann

docs/guide/migrating-to-v6.md

@@ -2,13 +2,23 @@
 
 ## Overview
 
-v6 is mostly a cleanup release with updated dependencies. The main changes are:
+`v6` is mostly a cleanup release, with updated dependencies. The main changes are:
 
-* Minimum required PHP version is now 8.2
-* ...
+* The minimum required PHP version is now 8.2
+* `radebatz/type-info-extras` is now a required dependency
+* `TypeInfoTypeResolver` now properly handles composite types (unions and intersections)
+* Some deprecations have been removed (see below)
+* The `MediaType::encoding` property now only accepts `Encoding` objects (BC break)
 
 For most installations upgrading should not require any changes.
 
+## Type resolvers
+With `radebatz/type-info-extras` now being a required dependency, the `TypeInfoTypeResolver` is not the de-facto default
+resolver.
+
+The `LegacyTypeResolver` can still be used as a drop-in replacement, but is now marked `deprecated` and will be removed
+in v7.
+
 ## Removed deprecated elements
 ### Methods `\Openapi\Generator::getProcessors()` and `\Openapi\Generator::setProcessors()`
 Use `getProcessorPipeline()` and `setProcessorPipeline(new Pipeline(...))` methods instead

src/Type/LegacyTypeResolver.php

@@ -12,6 +12,9 @@
 use OpenApi\Generator;
 use OpenApi\TypeResolverInterface;
 
+/**
+ * @deprecated use `TypeInfoTypeResolver` instead
+ */
 class LegacyTypeResolver extends AbstractTypeResolver
 {
     /** @inheritdoc */