Fix build failure from must_use docs

src/code-considerations/design/must-use.md

@@ -4,29 +4,29 @@ The `#[must_use]` attribute can be applied to types or functions when failing to
 
 As an example, `Result` is `#[must_use]` because failing to consider it may indicate a caller didn't realise a method was fallible:
 
-```rust
+```rust,ignore
 // Is `check_status` infallible? Or did we forget to look at its `Result`?
 check_status();
 ```
 
 Operators like `saturating_add` are also `#[must_use]` because failing to consider their output might indicate a caller didn't realise they don't mutate the left-hand-side:
 
-```rust
+```rust,ignore
 // A caller might assume this method mutates `a`
 a.saturating_add(b);
 ```
 
 Combinators produced by the `Iterator` trait are `#[must_use]` because failing to use them might indicate a caller didn't realize `Iterator`s are lazy and won't actually do anything unless you drive them:
 
-```rust
+```rust,ignore
 // A caller might not realise this code won't do anything
 // unless they call `collect`, `count`, etc.
 v.iter().map(|x| println!("{}", x));
 ```
 
 On the other hand, `thread::JoinHandle` isn't `#[must_use]` because spawning fire-and-forget work is a legitimate pattern and forcing callers to explicitly ignore handles could be a nuisance rather than an indication of a bug:
 
-```rust
+```rust,ignore
 thread::spawn(|| {
     // this background work isn't waited on
 });