Add speaker notes to bindgen slides (#2491)

Part of #1083.

src/android/interoperability/with-c/bindgen.md

@@ -73,3 +73,14 @@ _interoperability/bindgen/Android.bp_:
 ```shell
 {{#include ../../build_all.sh:libbirthday_bindgen_test}}
 ```
+
+<details>
+
+- The Android build rules will automatically call `bindgen` for you behind the
+  scenes.
+
+- Notice that the Rust code in `main` is still hard to write. It is good
+  practice to encapsulate the output of `bindgen` in a Rust library which
+  exposes a safe interface to caller.
+
+</details>

src/android/interoperability/with-c/hand-written.md

@@ -19,3 +19,10 @@ We already saw this in the
 
 > This assumes full knowledge of the target platform. Not recommended for
 > production.
+
+<details>
+
+- This is just a motivating example. For a real library, you want to use
+  `bindgen` as shown on the next slide.
+
+</details>