Add safety comments and use consistent format for existing ones. (#1981)

We should have safety comments on all `unsafe` blocks, to set a good
example.

src/android/interoperability/with-c.md

@@ -12,6 +12,7 @@ extern "C" {
 
 fn main() {
     let x = -42;
+    // SAFETY: `abs` doesn't have any safety requirements.
     let abs_x = unsafe { abs(x) };
     println!("{x}, {abs_x}");
 }

src/android/interoperability/with-c/bindgen/main.rs

@@ -20,7 +20,10 @@ use birthday_bindgen::{card, print_card};
 fn main() {
     let name = std::ffi::CString::new("Peter").unwrap();
     let card = card { name: name.as_ptr(), years: 42 };
-    // SAFETY: `print_card` is safe to call with a valid `card` pointer.
+    // SAFETY: The pointer we pass is valid because it came from a Rust
+    // reference, and the `name` it contains refers to `name` above which also
+    // remains valid. `print_card` doesn't store either pointer to use later
+    // after it returns.
     unsafe {
         print_card(&card as *const card);
     }

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

@@ -9,6 +9,7 @@ extern "C" {
 
 fn main() {
     let x = -42;
+    // SAFETY: `abs` doesn't have any safety requirements.
     let abs_x = unsafe { abs(x) };
     println!("{x}, {abs_x}");
 }