Improve description of Kotlin generated code

I'm not sure what the best wording is for this paragraph, but it's at least worth mentioning the generated `...OrNull` property.

Author: wfhartford

content/programming-guides/proto3.md

@@ -226,8 +226,9 @@ from an input stream.
     message type, as well as a special `Builder` class for creating message
     class instances.
 *   For **Kotlin**, in addition to the Java generated code, the compiler
-    generates a `.kt` file for each message type, containing a DSL which can be
-    used to simplify creating message instances.
+    generates a `.kt` file for each message type with an improved Kotlin API.
+    This includes a DSL which simplies creating message instances, a nullable
+    field accessor, and a copy function.
 *   **Python** is a little different — the Python compiler generates a module
     with a static descriptor of each message type in your `.proto`, which is
     then used with a *metaclass* to create the necessary Python data access