docs: add note to ElementExt::on about dropping the handle

[lukasschlueter]

Dropping the RemoveEventHandler returned by ElementExt::on also drops the closure, which invalidates the event listener.

Example code:

let button_ref: NodeRef<Button> = NodeRef::new();

button_ref.on_load(move |btn| {
    let cb = btn.on(leptos::ev::click, move |ev| {
        /* ... */
    });
    // Delay the drop to keep the event listener alive
    on_cleanup(move || std::mem::drop(cb));
});

[gbj]

Running cargo doc to test this out gives the following error:

warning: unresolved link to `reactive_graph::owner::Owner::on_cleanup`
  --> tachys/src/html/element/element_ext.rs:49:37
   |
49 |     /// Consider using [on_cleanup](reactive_graph::owner::Owner::on_cleanup) to delay dropping:
   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `reactive_graph` in scope
   |
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

and a broken link.

This is because the reactive_graph crate is not necessarily included but is an opt-in feature. Perhaps some text similar to the must_use message, or maybe just mentioning "If you're using this with Leptos, you can use on_cleanup"? I never know exactly how to handle these situations where the crate is designed for more general purpose use but there are Leptos-specific docs that would be useful.