Improve must_use comment on RemoteHandle

[Nemo157]

futures-rs/futures-util/src/future/future/remote_handle.rs

Line 38 in 3bf5ac9

#[must_use = "futures do nothing unless you `.await` or poll them"]

This is burying the lede, it seems more important to mention the behaviour on drop, which will cause many more issues if the handle is not used

futures-rs/futures-util/src/future/future/remote_handle.rs

Lines 23 to 24 in 3bf5ac9

	/// [`remote_handle`](crate::future::FutureExt::remote_handle). When you drop this,
	/// the remote future will be woken up to be dropped by the executor.