Make some comments doc comments

Co-authored-by: Dmitri Gribenko <[email protected]>

Author: tall-vase

src/idiomatic/leveraging-the-type-system/borrow-checker-invariants.md

@@ -8,8 +8,8 @@ The borrow checker, while added to enforce memory ownership, can be
 leveraged model other problems and prevent API misuse.
 
 ```rust,editable
-// Doors can be open or closed, and you need the right key to lock or unlock
-// one. Modelled with a Shared key and Owned door.
+/// Doors can be open or closed, and you need the right key to lock or unlock
+/// one. Modelled with a Shared key and Owned door.
 pub struct DoorKey {
     pub key_shape: u32,
 }

src/idiomatic/leveraging-the-type-system/borrow-checker-invariants/single-use-values.md

@@ -9,13 +9,13 @@ example of this is in cryptography: "Nonces."
 
 ```rust,editable
 pub struct Key(/* specifics omitted */);
-// A single-use number suitable for cryptographic purposes.
+/// A single-use number suitable for cryptographic purposes.
 pub struct Nonce(u32);
-// A cryptographically sound random generator function.
+/// A cryptographically sound random generator function.
 pub fn new_nonce() -> Nonce {
     Nonce(4) // chosen by a fair dice roll, https://xkcd.com/221/
 }
-// Consume a nonce, but not the key or the data.
+/// Consume a nonce, but not the key or the data.
 pub fn encrypt(nonce: Nonce, key: &Key, data: &[u8]) {}
 
 fn main() {