Add speaker notes and safety comment for raw pointers.

src/unsafe/raw-pointers.md

@@ -9,6 +9,10 @@ fn main() {
     let r1 = &mut num as *mut i32;
     let r2 = &num as *const i32;
 
+    // Safe because r1 and r2 were obtained from references and so are guaranteed to be non-null and
+    // properly aligned, the objects underlying the references from which they were obtained are
+    // live throughout the whole unsafe block, and they are not accessed either through the
+    // references or concurrently through any other pointers.
     unsafe {
         println!("r1 is: {}", *r1);
         *r1 = 10;  // Data race if r1 is being written concurrently!
@@ -16,3 +20,23 @@ fn main() {
     }
 }
 ```
+
+<details>
+
+It is good practice (and required by the Android Rust style guide) to write a comment for each
+`unsafe` block explaining how the code inside it satisfies the safety requirements of the unsafe
+operations it is doing.
+
+In the case of pointer dereferences, this means that the pointers must be
+[_valid_](https://doc.rust-lang.org/std/ptr/index.html#safety), i.e.:
+
+ * The pointer must be non-null.
+ * The pointer must be _dereferenceable_ (within the bounds of a single allocated object).
+ * The object must not have been deallocated.
+ * There must not be concurrent accesses to the same location.
+ * If the pointer was obtained by casting a reference, the underlying object must be live and no
+   reference may be used to access the memory.
+
+In most cases the pointer must also be properly aligned.
+
+</details>