Add note about AIDL methods taking &self

src/android/aidl/example-service/service.md

@@ -23,6 +23,15 @@ _birthday_service/Android.bp_:
 
 - Point out the path to the generated `IBirthdayService` trait, and explain why
   each of the segments is necessary.
+- Note that `wishHappyBirthday` and other AIDL IPC methods take `&self` (instead
+  of `&mut self`).
+  - This is necessary because binder responds to incoming requests on a thread
+    pool, allowing for multiple requests to be processed in parallel. This
+    requires that the service methods only get a shared reference to `self`.
+  - Any state that needs to be modified by the service will have to be put in
+    something like a `Mutex` to allow for safe mutation.
+  - The correct approach for managing service state depends heavily on the
+    details of your service.
 - TODO: What does the `binder::Interface` trait do? Are there methods to
   override? Where source?