update documentation for enableCppRenderSystem feature flag

Summary:
This diff updates the documentation for enableCppRenderSystem feature flag

This is a follow up of D38725771 (399907f)

changelog: [internal] internal

Reviewed By: cortinico, makovkastar

Differential Revision: D39558834

fbshipit-source-id: e079ee688a912dfa2670c66e5adf347a902a45e1

Author: mdvacca

Libraries/ReactNative/ReactNativeFeatureFlags.js

@@ -39,7 +39,8 @@ export type FeatureFlags = {|
    */
   animatedShouldUseSingleOp: () => boolean,
   /**
-   * Enables C++ render system.
+   * This feature flag enables an experimental render system that allows
+   * to render react components driven by classes written in C++.
    */
   enableCppRenderSystem: () => boolean,
 |};