Fix compiler docs

compiler/rustc_hir_analysis/src/check/check.rs

@@ -707,10 +707,12 @@ pub(super) fn check_opaque_for_cycles<'tcx>(
 /// check those cases in the `param_env` of that function, which may have
 /// bounds not on this opaque type:
 ///
-/// type X<T> = impl Clone
+/// ```ignore (illustrative)
+/// type X<T> = impl Clone;
 /// fn f<T: Clone>(t: T) -> X<T> {
 ///     t
 /// }
+/// ```
 ///
 /// Without this check the above code is incorrectly accepted: we would ICE if
 /// some tried, for example, to clone an `Option<X<&mut ()>>`.