Update AIBinder_setExtension docs

 *     When registering the interface, add:

 *         std::shared_ptr<MyFoo> foo = new MyFoo; // class in AOSP codebase

 *         std::shared_ptr<MyBar> bar = new MyBar; // custom extension class

 *         ... = AIBinder_setExtension(foo->asBinder().get(), bar->asBinder().get());

 *         SpAIBinder binder = foo->asBinder(); // target binder to extend

 *         ... = AIBinder_setExtension(binder.get(), bar->asBinder().get());

 *         ... = AServiceManager_addService(binder.get(), instanceName);

 *         // handle error

 *

 *         Do not use foo->asBinder().get() as the target binder argument to

 *         AIBinder_setExtensions because asBinder it creates a new binder

 *         object that will be destroyed after the function is called. The same

 *         binder object must be used for AIBinder_setExtension and

 *         AServiceManager_addService to register the service with an extension.

 *

 *     Then, clients of IFoo can get this extension:

 *         SpAIBinder binder = ...;

 *         std::shared_ptr<IFoo> foo = IFoo::fromBinder(binder); // handle if null