Document all used vendor extensions in specific generator (#11797)

docs/generators/java-play-framework.md

@@ -73,6 +73,15 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 |withXml|whether to include support for application/xml content type and include XML annotations in the model (works with libraries that provide support for JSON and XML)| |false|
 |wrapCalls|Add a wrapper to each controller function to handle things like metrics, response modification, etc..| |true|
 
+## SUPPORTED VENDOR EXTENSIONS
+
+| Extension name | Description | Applicable for | Default value |
+| -------------- | ----------- | -------------- | ------------- |
+|x-discriminator-value|Used with model inheritance to specify value for discriminator that identifies current model|MODEL|
+|x-implements|Ability to specify interfaces that model must implements|MODEL|empty array
+|x-setter-extra-annotation|Custom annotation that can be specified over java setter for specific field|FIELD|When field is array & uniqueItems, then this extension is used to add `@JsonDeserialize(as = LinkedHashSet.class)` over setter, otherwise no value
+
+
 ## IMPORT MAPPING
 
 | Type/Alias | Imports |