Add a paragraph to the lifetimeless module doc (#9312)

# Objective

The `lifetimeless` module has been a source of confusion for bevy users
for a while now.

## Solution

Add a couple paragraph explaining that, yes, you can use one of the type
alias safely, without ever leaking any memory.

crates/bevy_ecs/src/system/system_param.rs

@@ -1413,9 +1413,16 @@ macro_rules! impl_system_param_tuple {
 all_tuples!(impl_system_param_tuple, 0, 16, P);
 
 /// Contains type aliases for built-in [`SystemParam`]s with `'static` lifetimes.
-/// This can make it more convenient to refer to these types in contexts where
+/// This makes it more convenient to refer to these types in contexts where
 /// explicit lifetime annotations are required.
 ///
+/// Note that this is entirely safe and tracks lifetimes correctly.
+/// This purely exists for convenience.
+///
+/// You can't instantiate a static `SystemParam`, you'll always end up with
+/// `Res<'w, T>`, `ResMut<'w, T>` or `&'w T` bound to the lifetime of the provided
+/// `&'w World`.
+///
 /// [`SystemParam`]: super::SystemParam
 pub mod lifetimeless {
     /// A [`Query`](super::Query) with `'static` lifetimes.