Add speaker notes to interop with C slides (#2496)

Part of #1083.

src/android/interoperability.md

@@ -8,3 +8,15 @@ that you can:
 
 When you call functions in a foreign language we say that you're using a
 _foreign function interface_, also known as FFI.
+
+<details>
+
+- This is a key ability of Rust: compiled code becomes indistinguishable from
+  compiled C or C++ code.
+
+- Technically, we say that Rust can be compiled to the same [ABI] (application
+  binary interface) as C code.
+
+</details>
+
+[ABI]: https://en.wikipedia.org/wiki/Application_binary_interface

src/android/interoperability/with-c.md

@@ -24,3 +24,17 @@ We already saw this in the
 > production.
 
 We will look at better options next.
+
+<details>
+
+- The [`"C"` part][extern-abi] of the `extern` block tells Rust that `abs` can
+  be called using the C [ABI] (application binary interface).
+
+- The `safe fn abs` part tells that Rust that `abs` is a safe function. By
+  default, extern functions are considered unsafe, but since `abs(x)` is valid
+  for any `x`, we can declare it safe.
+
+</details>
+
+[extern-abi]: https://doc.rust-lang.org/reference/items/external-blocks.html#abi
+[ABI]: https://en.wikipedia.org/wiki/Application_binary_interface