Add documentation for CodegenDiscriminator (OpenAPITools#587)

Author: jmini

docs/migration-guide.adoc

@@ -11,6 +11,20 @@ Another approach to find breaking changes is to look at issue and pull requests
 * link:https://github.com/OpenAPITools/openapi-generator/labels/Breaking%20change%20%28with%20fallback%29[Breaking change (with fallback)]
 * link:https://github.com/OpenAPITools/openapi-generator/labels/Breaking%20change%20%28without%20fallback%29[Breaking change (without fallback)]
 
+=== From 3.1.x to 3.2.0
+
+Version `3.2.0` is a minor version of OpenAPI-Generator, in comparison to `3.1.x` it contains some breaking changes, but with the possibility to fallback to the old behavior.
+The default value of some options might change.
+Projects relying on generated code might need to be adapted.
+
+==== Model (all languages)
+
+In `CodegenModel` and in `CodegenOperation` we use now our own class `org.openapitools.codegen.CodegenDiscriminator` instead of `io.swagger.v3.oas.models.media.Discriminator`.
+
+For the templates, this is not an API change, because the same values are available.
+
+If you have your own `Codegen` class (to support your own generator for example) then you might get some compile error due to the change.
+
 === From 3.0.x to 3.1.0
 
 Version `3.1.0` is the first minor version of OpenAPI-Generator, in comparison to `3.0.3` it contains some breaking changes, but with the possibility to fallback to the old behavior.