Add visibility to modeling guide

Author: delvedor

docs/modeling-guide.md

@@ -575,3 +575,18 @@ class Foo {
   faz: string
 }
 ```
+
+#### `@visibility`
+
+You can mark a request as `public`/`feature_flag`/`private` with this tag (the default is `public`).
+
+```ts
+/**
+ * @rest_spec_name namespace.api
+ * @since 7.5.0
+ * @visibility private
+ */
+export interface Request extends RequestBase {
+ ...
+}
+```