Mark Task as #[must_use] (bevyengine#6068)

The `async_executor::Task` that it wraps is also `#[must_use]` with the same message.

Co-authored-by: devil-ira <justthecooldude@gmail.com>

crates/bevy_tasks/src/task.rs

@@ -14,6 +14,7 @@ use std::{
 /// Tasks that panic get immediately canceled. Awaiting a canceled task also causes a panic.
 /// Wraps `async_executor::Task`
 #[derive(Debug)]
+#[must_use = "Tasks are canceled when dropped, use `.detach()` to run them in the background."]
 pub struct Task<T>(async_executor::Task<T>);
 
 impl<T> Task<T> {