[codegen] change x-oneOf-name to x-one-of-name. Consistency with naming conventions and x-all-of-name (#5820)

* change x-oneOf-name to x-one-of-name.

* Add code comments

* Add code comments

Author: sebastien-rosset

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -1857,7 +1857,13 @@ public String toAnyOfName(List<String> names, ComposedSchema composedSchema) {
     }
 
     /**
-     * Return the name of the oneOf schema
+     * Return the name of the oneOf schema.
+     *
+     * This name is used to set the value of CodegenProperty.openApiType.
+     *
+     * If the 'x-one-of-name' extension is specified in the OAS document, return that value.
+     * Otherwise, a name is constructed by creating a comma-separated list of all the names
+     * of the oneOf schemas.
      *
      * @param names          List of names
      * @param composedSchema composed schema