Mark .id() methods which return an Entity as must_use (#3750)

# Objective

- Calling .id() has no purpose unless you use the Entity returned
- This is an easy source of confusion for beginners.
- This is easily missed during refactors.

## Solution

- Mark the appropriate methods as #[must_use]

crates/bevy_ecs/src/lib.rs

@@ -1130,18 +1130,12 @@ mod tests {
     #[test]
     fn remove_bundle() {
         let mut world = World::default();
-        world
+        world.spawn().insert_bundle((A(1), B(1), TableStored("1")));
-            .spawn()
-            .insert_bundle((A(1), B(1), TableStored("1")))
-            .id();
         let e2 = world
             .spawn()
             .insert_bundle((A(2), B(2), TableStored("2")))
             .id();
-        world
+        world.spawn().insert_bundle((A(3), B(3), TableStored("3")));
-            .spawn()
-            .insert_bundle((A(3), B(3), TableStored("3")))
-            .id();
 
         let mut query = world.query::<(&B, &TableStored)>();
         let results = query

crates/bevy_ecs/src/system/commands/mod.rs

@@ -372,6 +372,7 @@ impl<'w, 's, 'a> EntityCommands<'w, 's, 'a> {
     /// # my_system.system();
     /// ```
     #[inline]
+    #[must_use = "Omit the .id() call if you do not need to store the `Entity` identifier."]
     pub fn id(&self) -> Entity {
         self.entity
     }

crates/bevy_ecs/src/system/mod.rs

@@ -493,8 +493,8 @@ mod tests {
         world.clear_trackers();
 
         // Then, try removing a component
-        world.spawn().insert(W(3)).id();
+        world.spawn().insert(W(3));
-        world.spawn().insert(W(4)).id();
+        world.spawn().insert(W(4));
         world.entity_mut(entity_to_remove_w_from).remove::<W<i32>>();
 
         fn validate_remove(

crates/bevy_ecs/src/world/entity_ref.rs

@@ -26,6 +26,7 @@ impl<'w> EntityRef<'w> {
     }
 
     #[inline]
+    #[must_use = "Omit the .id() call if you do not need to store the `Entity` identifier."]
     pub fn id(&self) -> Entity {
         self.entity
     }
@@ -113,6 +114,7 @@ impl<'w> EntityMut<'w> {
     }
 
     #[inline]
+    #[must_use = "Omit the .id() call if you do not need to store the `Entity` identifier."]
     pub fn id(&self) -> Entity {
         self.entity
     }