Release notes on non-text detail arguments. Closes encode#2341.

docs/topics/3.0-announcement.md

@@ -940,6 +940,7 @@ The default JSON renderer will return float objects for un-coerced `Decimal` ins
 * The serializer `ChoiceField` does not currently display nested choices, as was the case in 2.4. This will be address as part of 3.1.
 * Due to the new templated form rendering, the 'widget' option is no longer valid. This means there's no easy way of using third party "autocomplete" widgets for rendering select inputs that contain a large number of choices. You'll either need to use a regular select or a plain text input. We may consider addressing this in 3.1 or 3.2 if there's sufficient demand.
 * Some of the default validation error messages were rewritten and might no longer be pre-translated. You can still [create language files with Django][django-localization] if you wish to localize them.
+* `APIException` subclasses could previously take could previously take any arbitrary type in the `detail` argument. These exceptions now use translatable text strings, and as a result call `force_text` on the `detail` argument, which *must be a string*. If you need complex arguments to an `APIException` class, you should subclass it and override the `__init__()` method. Typically you'll instead want to use a custom exception handler to provide for non-standard error responses.
 
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