Force the compiler and clippy to return a warning when a function isn't documented

src/kernel/system.rs

@@ -30,6 +30,7 @@ impl<'a> System<'a> {
         }
     }
 
+    /// use thread_manager setter to send it system instance
     pub fn freeze(&'a mut self) {
         self.thread_manager.system.set(Option::Some(self));
     }